gmaxwell explained it well
They've been assigned in blocks in an effort to keep related BIPs together.
Also, sometimes people have ignored the process and self-assigned numbers and started using them in communications-- sometimes multiple people with the same number, when that happens the number gets temporarily skipped for assignment to avoid adding confusion. But primarily just due to grouping.
BIPs from #1 to implement whatever I've missed
You absolutely shouldn't do that. The BIP process has virtually no editorial control-- it's just a publication numbering scheme that assigns a number to anything that persists long enough requesting one. There are many low quality / broken BIPs that no sane party should use.
Essentially the only editorial lever in BIPs is that if many people dislike a proposal they'll encourage the proposer to abandon it before it reaches the point of getting a number assigned. If the proposer can't be convinced or if no one cares enough to convince them and the proposer persists their proposal will get numbered.
TLDR : BIP number it's not used in sequence, but by category
