design a 90 degree hybrid coupler 2-8GHZ
Generally, I can only assume that these mismatches stem from the effects of asymmetry. Note that your &lt;4.5 dB mismatch doesn't look all that bad!

you could ever so slightly adjust the angle of your feeds and see where it takes you, but you'll probably lose a bit of your nice S11 that way.

Other people avoid asymmetry by actually making their ring round: 



However, I have no clue how well you can achieve such a wideband match as you did with that.