How/why helpfulness ratings on reviews are fluctuating downwards?

There are only 2 reasons I can think of this would be happening.  The person's account is deleted or later people who upvoted, then downvote to cancel out previous vote(which I don't know if this is even possible).  

What I'm noticing is that movies or series I have reviewed would all of a sudden, in a matter of a day or 2, drop by multiple overall votes.  I know of at least 2 specific TV Series that this has occurred on.  The TV series "From" and "Invasion".  I noticed this first on the series "Invasion", in a day's time the overall count dropped from either 11 or 12 to 8.  "From" was a similar drop in number, but I don't recall the exact total number(I believe it was previously at 8 total votes, down to 3).  On the series "From" this could have occurred over a 2 day time span, but I'm positive on "Invasion" that the drop was in a single day.

So what is going on with the voting?  Why are votes being gobbled up?  I have never noticed drops like that previously.

And no, in the grand-scheme of things it probably doesn't really matter.  There does seem to be some kind of issue though.