increasing b-frame is good or not?

[huang_ch]

I'm trying x264 these days, and I've read some posts here which mentioned that increasing b-frame to above 3 may be useless for the encoder itself may hardly choose to use b-frame above 3. But in a recent try, I've found that by increasing b-frame to 16, the final bitrate is decreased a lot and PSNR also decreases.
All uses crf24, the raw file is a anime
1. b-frame 16: final bitrate: 288Kbps, PSNR: Avg:45.054 Global:44.097
2. b-frame 3: final bitrate: 335Kbps, PSNR Avg:46.512 Global:45.068

Is this the expected behavior? Increasing b-frame will reduce the bitrate but also PSNR? Though I didn't see many major loss with my eye....