I think the iPhone takes it's elevation from something like Google maps etc. The reason I say this is that I live in south Florida and on my weekend ride I have one "climb" of about 75 feet over a drawbridge and Strava shows 0 feet of climbing. I assume that Google maps or whatever the source is is thinking the road goes straight and level across the Intercoastal. I have a GPS compass app on my phone and I have stopped and taken a reading and it shows my elevation at 73 feet.