Crypto exchanges algorithms
you're providing alpha to "high frequency market makers". Since they can tell that you are not a bot, they are assuming its a real bid, so they can bid on top of you and then they are now in a risk free position. 

if they get hit, they will immediately place an ask at a price of:  last + (last-bid). they will wait hoping you will pay up, they may even entice you by placing another bid higher... then (within a couple of seconds) they will hit your bid, and take the small loss. 

as long as the spread is much wider than the tic above you, they are in positive alpha. 

in general, you should not try to play this game. just place "marketable" limit orders.. so buys at the best ask, and sells at the best bid, otherwise you are giving away your hand.