Go take a look at @reggaemuffin 's comment/suggestion below, ref:

https://steemit.com/steem/@ura-soul/the-most-voted-witness-in-the-last-3-and-6-months-this-is-what-witness-vote-expiry-decay-would-look-like-kinda#@reggaemuffin/re-jackmiller-re-reggaemuffin-re-ura-soul-re-reggaemuffin-re-ura-soul-the-most-voted-witness-in-the-last-3-and-6-months-this-is-what-witness-vote-expiry-decay-would-look-like-kinda-20181006t120630520z

and my response to it.

This line of thought that he brought up could actually be an answer to many things, including but not limited to:

dead voters (& in turn dead followers)

• RC's that are just sitting there and not being used!

;)