Ahh yes this is something that only happens once your account reaches a certain level of steem power, now it is not as important for lower strength votes to be broken down into smaller increments - but as the power grows so does the strength of the vote and the payout, whales don't want to use all of their power at once if they break it up into increments more users can be rewarded.
When you do get enough power each time up upvote a post or comment, the slider will appear and you can then select the percentage you wish to apply.
Hope this helped answer your question, let me know if you need more info :)
Thanks for the answer. I'm curious as to how much steem power I need to get that slider?
I believe you need to get to around 500 steem power, at least that is what it was 9 months ago - I haven't heard of any changes there.. Cheers :)