Again, I would suspect that the asymmetry results from something in your measurement set up. I note you are suing end fed dipole surrogates (flat PCB) elements. I gather this is a variation of a Franklin antenna. Are you going for broadside peaks in the pattern? End fed dipoles are notorious for putting undesired currents along the feed line. Radiation from a feed line can muck up the works. More opportunity for asymmetry in the measured patterns. Run your hand along the feed and if you see variation in s11 the line is "hot" and probably radiating. You might look in the H plane to see how omnidirectional the pattern really is. I'll bet you have frequency squint in the E plane pattern too.
That may not be an issue, depending on your requirements.
Regards,
Azulykit