IMHO, generally it happens when the fort is build to exactly 100%, so you get fort level 1 and 0% towards level 2. Or you get fort level 2 and 0% towards level 3.
Now, any damage to fort will result in its reduction by one level (which will be level 1 in case of fort 2 and no fort in case of fort level 1). This damage may be caused by artillery, sappers etc. As such, it is difficult to tell whether it is more prudent to display cv assuming that the fort will be intact during battle, or actually display the cv assuming the fort will get destroyed. Note that in case of your units, the fort will most likely (if you have any mp left it is almost certain) build a bit in the logistic phase (so there will be some % towards next level), so your opponent will face proper fort 1 and proper fort 2. In such case the opponent will see the numbers correctly.
If you see opponents fort without corresponding increases in the cv on the counters, it means that the fort is exactly at 0% towards next level and it is likely that it will be reduced during combat. So the cv numbers you see (lower than the fort level suggests) will be likely those which will count in the end of the battle. In practice, I have experienced battles where fort 1 0% survives - and after the attack the cv numbers on the counters are properly displayed ;)
Hope it helps,