Edit: Taken from PC Hardware in a Nutshell:
"The processor clock coordinates all CPU and memory operations by periodically generating a time reference signal called a clock cycle or tick. Clock frequency is specified in gigahertz (GHz), which specifies billions of ticks per second. Clock speed determines how fast instructions execute. Some instructions require one tick, others multiple ticks, and some processors execute multiple instructions during one tick."
The time between ticks is determined by your clock speed, and it takes one to many ticks depending on the OP being performed. For example, a 286 class CPU needs 20 ticks to multiply two numbers.
If you need high performance timers, then I don't think you can rely on ticks being constant across all systems.
The CPU scheduler could have delayed the thread, especially if there was another thread with a higher priority. So yes, the CPU could've been too busy.