Many battery-powered " sleepy" devices reports battery levels in tiers. The exact tiers will vary by device.
For most contact sensors, the tier that is reported as “88%” really means “above 87% and below 100%”
So as soon as battery level drops below 100%, even to 99%, it will be reported by SmartThings as 88%.
What you need to watch is the trend. If it drops 10% in one day. That was probably just a tier drop. You can even bounce back-and-forth by 10% because it’s right at the threshold of a tier.
But if it’s dropping 10% every week, that’s a different problem.
If you’ve had any device off-line for a while, it’s really common for it to come back in the next lower tier, which looks like a 10% drop but might’ve been a drop of half a percent. Or might just mean it hadn’t reported for a while previously.
Also note that lithium batteries do not have the same drain pattern, which throws reporting off. They commonly look about the same as other batteries down to about 50%, and then drop very rapidly after that. For this reason, most lock manufacturers recommend not using lithium batteries, as you may not get a low battery warning in time to replace the batteries.
Also some devices only report noncritical battery levels once a week unless specifically polled. Again, that’s to make the batteries last longer. This can also result in apparent sharp drops.