I really should make a post on this - and will.

Basically, with each vote, you lose some of your voting power ~ 2%. It takes about 2 + hours to get back up to 100%. If you vote 10 times, your voting power is at 80% and it takes 24 hours to get back up to 100%.

Every time you lose voting power, your vote is worth less - even though you are voting at 100%. That means that you give less to the person you are voting for and also get less in curation rewards. Especially if you don't have much steempower, if your voting power gets too low - your vote doesn't benefit you much. If you go to steemworld.org you can see what is going on with your account :) And if you are below 500 steempower, by default, all your votes are 100% votes.