I had posted this issue I have with my KC901 before but the results were not displayed in this manner which emphasizes the difference. As I said previously, all the other resolution bandwidths 10, 3 & 1kHz are symetrical around the centre frequency as expected. For 30kHz RBW setting, the side of the response that rolls away quicker is only about 6.9kHz at its 3dB point. The measured 3dB bandwidth of the filter, ignoring that it is offset, is about 24.5kHz.
For this test I used my R&S CMT54 test set to provide a 500MHz carrier at 0dBm and then observed it with the widest 30kHz bandwidth. The KC901V span is set to 50kHz. This effectively traces the IF bandwidth response which I then stored with the hold feature. I then pressed the Lo Hi/Low button which effectively flips the IF response. The markers are set at the +/- 15kHz points so you can see that the response is way out either way you look at it. The top of the response is also about 0.8 dBm not 0dBm.
I expect the issue is a hardware one requiring a change of the pre-selection filter but there might be another cause and I would be happy to know what solutions might be available.