A problem which can be seen quite often on the analog gauges, not only CS ones.
Mainly a problem of the actual time a new number "rolls over" to show your current altitude.

Being at e. g. 12900ft might give you 13900 showing up because the thousands already rolled while the hundreds are behind.
This happen here and there, so coming down from 10.000 and going to 9.900 could be a problem too.
Strange fact: as some digital gauges use the rollover effect too, they suffer from the same problem which really becomes one if you stay at that altitude where the rollever wants to happen but can't because you are not descending or climbing.

Might be that I got used to this flaw when climbing or descending on the 727 but I had it lately on another plane where I tried to stay at 13.000 and got "13900" all the time (the "3" already was fully established, no sign of the still 10% active "2").
So the actual test scenario is one where the thousands or ten thousands should roll over to a new number. The direction doesn't matter, just try to fly at e. g. 10.000 or 20.000 and watch the numbers when going slightly up or down.

Maybe some of this behavior is intended (for some unknown reason) but I usually expect it to happen like on an old automobile. The numbers roll over slowly, so 25.000 miles or something give you a half view on the 2 and the 3 on the first digit.