You're doing the right thing in looking in detail and especially including the full specs, but it can be frustrating as so much seems contradictory and confusing. Some of the basics that might help narrow things down, these are generalisations(you can still find opinions that differ
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) but are my take from my research:-
Frame material - aluminium gives a harsher ride but if you're getting suspension this is negated, steel is loved by many, Ti and carbon are great but pricier and carbon isn't as prevalent in all round style bikes
Tyres - the size of tyre, MTB 26" or road 700c is a key choice in a frame and also, for 700c, the max tyre you'd want to run. Many road frames will limit you to 700x25/28 as a maximum. CX bikes have become popular/trendy and it's a lot to do with people wanting a roadish type ride but not a full on touring rig up, that takes bigger tyres. As I've already mentioned, if the bike has disc brakes then 26" wheels can be run as well as 700c, if it's a 700c frame.
Brakes - if you want bigger tyres then you're limited to very deep drop dual pivots, cantis, v-brakes, cable disc or hydraulic discs. The first two are short pull so require short pull levers and will work with most road levers and STI type shifters. V-brakes are long pull(you can get mini v's but I'm not going there today) cable discs can be had in short or long pull, most notably the Avid BB7 road or MTB versions. Hydraulics speak for themselves but they don't fit on drop handlebars.
Handlebars - road bars have a larger diameter than flat bars so MTB controls/levers don't slide on. If you buy a bike and then want to change from flat to drops, or vice versa, it generally works out expensive. There are alternate handlebars that offer multiple hand positions but they are generally considered deeply uncool, hence I'm a big fan
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Most people tend to say, flat bars fine but no good for longer rides due to lack of hand positions, so go for drop bars, but there are options in between.
Geometry - the effective top tube on a road/CX frame will normally be shorter than a MTB/29er/flat bar frame, though some flat bar road bikes have the shorter geometry. This is to account for the fact that drop bars increase the reach to the main hand areas, this amount varies depnding on the bars but 60mm would be short and 80-90mm more normal. For example a road frame for me would have an effective TT of about 570mm whereas a 29er frame is about 620mm.
Componentry - you can't just mix and match gearing, front and rear mechs, shifters etc, you need to check what works with what. Shimano 9 speed and less mixes pretty well across road and MTB but the newer stuff is getting trickier.
Frame fittings - do you want to be able to fit mudguards and a rack, or even a front rack - these things help narrow down bike/frame choices as well
Suspension - nothing wrong with it but at the cheap end of the spectrum it really just takes away energy and wouldn't be up to the rigours of full on MTBing, it just adds weight for little gain. But there are plenty that are happy to sacrifice a bit of efficiency to make a commute comfier over rough roads, depends on length of commute etc. Most people on here would probably recommend going fully rigid on a road/trail use bike. For example the new 29er I've built is fully rigid as I don't expect to be doing the sort of riding that needs suspension, if I was then I'd be looking at £400+ for forks alone.
The problem is that so much of this depends on the individual and the type of riding they will do, which is fine but is generally an unknown quantity for a newish rider. A lot of bike shops recommend based on what they need to shift or on their experience rather than what you say. So they might look at you and think 'pootle with the kids, odd 10 miler, any old bike will do'. My gut reaction would be to go for versatility now and, as you develop an understanding of your preferences, spec a more suitable bike later. If you get a decent versatile bike now then the chances are that would be a keeper as a commuter/all-rounder and a future bike as a longer weekend type ride.
I could go on but you get the gist and hopefully understand why 'what bike?' is such a hard question to answer. Remember most bikes that the bike shops sell get very little use so, in that respect, their any bike will do attitude is correct. Though it could be argued that selling a really wrong sort of bike actually puts people off riding.