Bike computers -- GPS, non-GPS, wired, wireless, smartphone apps, dedicated computers, magnet vs earth induction sensors -- remind me of my early lessons in photography and photometry: Use three different light meters, get four different readings and, if you ask three other photographers for corroboration, nine different opinions.
Or something like that.
Whenever I add a cycling device or try a new app, I'll usually run two or three different devices and/or apps simultaneously for a few rides until I'm satisfied with the new device/app. They all differ a bit, and all are close enough.
I've compared -- on the same rides -- my old iPhone 4s (with ANT+ dongle), newer Android phone (Bluetooth only), both running Strava and Wahoo Fitness, and a cheap but good wireless computer using front wheel magnet sensor (non-GPS, non-Bluetooth/ANT+), along with a new GPS computer that supposedly uses all global positioning protocols (no way to select, it chooses automagically) along with both Bluetooth and ANT+ sensors.
Yeah, they all differ a bit. Especially after uploading to Strava, which massages all our data using its own secret sauce anyway. But they're all close enough.
I've never been close enough to taking any significant KOMs for the few nths of a second or milliparsecs to matter. Maybe if I was within 1 second of a worthwhile KOM I'd be concerned. But that's never gonna happen, not even in my age group.
A few years ago there were occasions when my iPhone with Strava had significant glitches in distance and time/speed. Difficult to say whether it was only Strava's fault, only my phone's fault, or a combination of both -- maybe with some weather or other interference thrown in. Many of y'all probably remember getting those inexplicable straight-lines across open fields, through buildings, etc., when we'd lose GPS sync, then regain it a few miles or minutes later.
I'm not sure that doesn't happen anymore, but it appears some apps do a little intelligent guesstimating about our probable route and fill in the blanks. Cyclemeter was the first app I used that seemed to do that. If I ran both Strava and Cyclemeter, Strava might glitch and show a straight line across a huge pasture or middle of a highway until it regained sync, while Cyclemeter would reconstruct my actual route with remarkable accuracy -- possibly based on my history of riding the same or similar routes.
When that happens, our data can only be an approximation.
And even riding the same route repeatedly, varying no more than a few feet here and there depending on how wide I take a turn, or whether I needed to dodge a snake or skunk, there are minor variations but nothing really significant.
Same with my bike computer's built in thermometer and barometric/elevation sensors. They're kinda sorta handy, but mostly just confirm data available through other sources. Elevation tends to be essentially identical to Strava and Google. Ditto temperature. The main thing the bike computer's thermometer is good for is to show the effects of micro-climates, those sudden temperature changes we feel some days/nights, dipping into low spots, then rises, on roller coaster terrain -- especially along rivers or lakes, even dry creek beds that serve as conduits for cooler air. And both sensors can be thrown off by other variables -- direct sunlight on the bike computer, humidity and weather fronts that affect barometric pressure.
Still pretty impressive technology for $50 or less, which is all I paid for my multi-function GPS computer.