Hmm, I get that. In Medibang you could literally see the pixels to the max if you zoom it and on ibis the pixels is pretty much got smoothened so it doesn't looks pixelated too much. That's also one thing I like from ibis, since my obsession with perfect lines got a little bit toned down. XD
But I use Medibang most of the time since they offer a maximum of 20000px for my comic making. I just hate they don't really have the option to changing color like ibis (they does have it but it's a bit different).