Yes. Strava just takes the datapoints provided by the device. The Garmin will be a lot more accurate.
Depending on the phone, a Garmin may be more accurate but GPS itself is not inch-perfect.
According to
this site, under normal conditions the worst-case-scenario can have your position out by 7.8 metres.
For example:
You stand still and your GPS device reads out by 7.8 metres to the South.
1 second later, it polls your position again and is out by 7.8 metres to the North.
It then thinks you have moved 15.6 metres in that second.
15.6m/s = 35mph, whilst not actually moving.
This is the inaccuracy in the signal, before taking into account inaccuracies in the device (rounding errors, low processing speeds, inaccurate timing, not having a lock on enough satellites, etc).
It's possible to apply various methods to improve accuracy. Clever maths or combining GPS readings with accelerometers are effective at getting very accurate readings.
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