I suspect they use a different length of time in seconds before flagging you as having stopped moving. The Garmin would naturally be more generous and stop almost instantly. Strava being used with so many low quality phones will need to use a longer time period.
Not necessarily, because distances can be different too, the biggest difference is in elevation, though strava tends to give more on the 705 than the 705 records, I still use the rwgps corrected even if its lower. There are also usually minor differences between my Rider 20 and the 705, even though both are started and stopped with a second or two of each other. The R 20 has no auto pause whilst the 705 does, today for instance
R20, ride time 2:10:49, 34.32 miles @ 15.7mph elevation 807ft
705, ride time 2:11:04, 34.31 miles @ 15.7mph elevation 1250ft
Both using the same sp/cad sensor and the same wheel circumference for a 25mm tyre.
The main difference is the R20 records every 3's the 705 on smart recording, in other words when something changes, I would set it to every second but it had a habit of switching off mid ride.
Edit for the record rwgps 1325ft and strava 1361ft.
Starva times 2:11:45 34.3 miles and 15.6mph. data from the 705.