In general, a cache manager SHOULD keep your entries for as long as possible, and MAY delete them if/when necessary.
The Time-To-Live (TTL) mechanism exists to flag entries as "expired", but expired entries are not automatically deleted, nor should they be, because APC is configured with a fixed memory size (using apc.shm_size configuration item) and there is no advantage in deleting an entry when you don't have to. There is a blurb below in the APC documentation:
If APC is working, the Cache full count number (on the left) will display the number of times the cache has reached maximum capacity and has had to forcefully clean any entries that haven't been accessed in the last apc.ttl seconds.
I take this to mean that if the cache never "reached maximum capacity", no garbage collection will take place at all, and it is the right thing to do.
More specifically, I'm assuming you are using the apc_add/apc_store function to add your entries, this has a similar effect to the apc.user_ttl, for which the documentation explains as:
The number of seconds a cache entry is allowed to idle in a slot in case this cache entry slot is needed by another entry
Note the "in case" statement. Again I take this to mean that the cache manager does not guarantee a precise time to delete your entry, but instead try to guarantee that your entries stays valid before it is expired. In other words, the cache manager puts more effort on KEEPING the entries instead of DELETING them.