A few of the Garmin's now upload through your phone, so negates the need to connect to computer also.
Have had a few discussions about Garmin's showing greater elevation on those with a barometric sensor. Mainly down to Strava elevation data being wildly inaccurate outside of the US. Down to the elevation data being only accurate to within 50-80 metres in the UK!
There's a segment close to me, which Strava shows as having a 7ft total elevation. Segment is 0.5 miles long and starts and finishes at rail track level and, you guessed it, goes over the track via a bridge on this busy bypass section. Has anyone seen a train for trough a tunnel that's only 7ft tall including the bridge thickness and road above it?
On the flip side, the elevation data on a number of mates' Garmin's, including mine sometimes, shows different elevation for the exact same route on different days. This is down to temperature and wind direction affecting the readings. I usually put mine outside for a while before using, to get the most accurate readings.
I very rarely saw any issues with Distance or Speed variance when running Garmin vs iPhone App when I first got the Garmin though.