b18 header?

i have a 90 b18a block with a 93 b18a head. i see on some sites like DC, where it said the header only fits 90-91 or 92-93. why wouldnt a 93 header work on a 90? i got my swap done at a shop so i dont know which stock header they used. but could anybody tell me the difference between these headers, i thought b series headers were interchangeable?

prolly are it might be runner length from the ports to the bend.

All b series headers fit any b series. What matters is the placement of the 02 sensors.