Old implementations of sleep just woke up once every second (inside the operating system) so saying sleep(1) means "wake me up the next time you wake up", which will be in something less than one second (almost one second if it had just recently woken up and almost immediately if it has been almost a second since it woke up).

Update: After chatting with Adam, he had one case that can't be explained by this. That was when his display ended in ":00" for quite a while and then shifted to ":59". The only explanation I can come up with for that is that the system clock isn't consistantly running at the same speed as the "sleep metronome". This is often true on systems where the "sleep metronome" is based on the "uptime clock" (which monotonously marches forward at something close to but not quite exactly real time) while the "time of day clock" has various types of adjustments applied to it in an attempt to keep it in sync with the major television networks.

        - tye (but my friends call me "Tye")