Different netcodes. I find that DOA5's online is either really great or really awful. 5 bar matches are essentially perfect and are able to replicate offline matches. 4 bar matches are passable but still a bit laggy. Anything lower can be just painful to play. If you have 3 bars or lower, don't bother to try and punish anything, you're just asking for CH after CH from mindless mashers.
I try my best to get in lengthy sessions with 5 bar connection players because that practise tends to translate offline. With 3 bars or less, you need to throw the rule book out the window and treat the match as a contest of who can press the P button fastest.