Inconsistent Web Page Counts In Google

When researching keywords, one of the first steps I take is to look at the total number of web pages that contain my target phrase, I realize this is not an indicator of who is actually competing but it helps me to get an overview.

Recently I noticed that the number of pages found in the index for one phrase increased dramatically. I can understand a sudden drop due to an index purge… but a sudden increase?

Within 2 months this phrase went from 430K to 9M, then 2 weeks later to 58M (broad match). Excluding the possibility of a sudden popularity increase, what are some other reasons for this seeming inconsistency?