Browsers tend to have a default font size of 16px and the Vector and Monobook skins use various techniques to end up at 12.8px resp. 12.7px for the bodytext.
Now, in commonElements.css, we have 105% for <h5> and 100% for <h6>. This gives rounded values of 13px for <h5> and <h6> in Vector and Monobook, making them hard to distinguish. (To avoid this, 108% has been used at dewiki for several years.)
Modern is not affected by this problem because it uses browser default font sizes for <hX>. Haven't looked at Cologneblue.