Soapy smear on dark bathroom tiles

[Amaaazing]

When I wipe down my dark brown bathroom wall tiles, with a sponge that I have rinsed out, a white soapy residue is left on the tiles, even though there is no cleaning product on the sponge.

What am I doing wrong?

Sometimes it's near damn impossible to get all soap out of a sponge. Only other thing I can think if is hard water (cannot completely say as I have always lived in a v. soft water area)