There are a few possible reasons out there:
1) Google states "Inactive keywords don't have a high enough Quality Score and Max CPC to trigger ads. These keywords may occasionally accrue clicks as we re-assess their quality."
2) Some people believe that there is still effectively an 'In Trial' keyword state, as inactive keywords seem to be behaving much like this supposedly nonexistent state.
3) It is possible that your keywords are being reviewed and changing states without your noticing-- going from inactive to active, running for a short while, then falling back into inactive.
Whatever the reason, there seems to be no way to ensure that a keyword will remain inactive (even with bids well below the minimum) without completely removing them from your campaign. Some people like this, though, as they have been able to get impressions at ridiculously low bids for competitive keywords, even if only part-time.
Hope this helps.
