Sometimes you create something, take a look at it later again.. and discover that you messed it up. In this case, it was about tiles not matching. The different color areas need to be properly aligned. The picture below shows the outer tiles all having the darker area taking 14-15 Pixels and the lighter taking the rest. Do not ask me why I did not go for 16/16.
The middle tiles, however, have got bigger dark color areas ( up to 17 pixels ). This lead to the following:
Next point on my list was variety. Done that. Added top, bottom and corner tiles up from 26 to a total of 50 different tiles.