I don't fully have it grasped myself, but the more sp u have the more your vote is worth. it won't effect the percentage or rate of recharge, it will effect what percent you can manually vote at though. On nanocheeze I can cast 6% votes, but on xtdevelopment I can only do 15% as the lowest... But you prolly don't know what I mean by a manual percentage vote, so... lol
But to answer your question, the rate should only differ if you are trading SBD for Steem using the internal market... As then you are literally trading, like a crypto currency which is kinda like trading stocks. so that would differ. but, when you power up steem straight to steem power it should be 1:1 ratio for sure...
Thank you, that helped a lot, keep steemin.