This comes up a few times a month. It's "normal," if by normal we mean common.
It's due in part to GPS sampling, how the apps use the data, and, with Strava, how they interpolate data and blenderize it with their own secret sauce.
Weather may be a factor too. There was some glitch Monday that dropped the first 15 miles of my 50 mile ride, using my bike computer. But my phone app logged the entire trip. However the average speeds, etc., were wildly different. That glitch seems to occur pretty often in the same area, so it may be radio frequency interference or EMI, or might be due to the shallow valley in that route, surrounded by adjacent hills, tree cover, etc.
It also depends on how the device or app uses auto-pause/resume. Some have an adjustable threshold, which may help with interruptions for traffic lights or stop signs for cyclists who slow roll, then accelerate. Others have a preset, non-adjustable threshold for auto-pause/resume, and our times, speeds and distances will differ.
If you watch some videos of cyclists tackling Strava segments, they'll often manually start and stop their computers, rather than relying on GPS for designated begin/end points to automatically mark their efforts. I'm not sure it's any more valid for the cyclist to start/stop computers, since it's bound to vary slightly from the actual begin/end points.
A wired computer, properly programmed for wheel diameter, etc., will probably give you the most accurate data, followed by wireless. My various Bluetooth and ANT+ sensors glitch occasionally, presumably due to RFI/EMI, so those aren't 100% accurate either.
Ditto GPS based estimates for speed and distance. Glitches can occur. Strava and some apps attempt to correct minor glitches when GPS sync is lost, and will try to correct the route trace by guesstimating our most likely route based on where we were when GPS sync was lost and reacquired.
Before getting a dedicated bike computer with GPS and compatibility with my various ANT+ and Bluetooth sensors, I'd occasionally carry an iPhone, an Android phone and run 2 or 3 apps to compare data. They always differed slightly.
After getting a bike computer I still run Strava or Wahoo Fitness to compare results. They always differ a bit from the bike computer's numbers using various sensors.
I'll worry about it if I'm ever fast enough to contend for a significant KOM. I've had a few top tens but those personal best times were still pretty remote from the KOMs, so minor discrepancies in my speed/time weren't significant.