I got this (use16 code) from NDISASM:

250F00 and ax,0xf

Recompiled with FASM.

Disassembled again:

83E00F and ax,byte +0xf



What's the problem ? Why does FASM assemble to different code ?
What does the strange syntax "byte +0xf" mean ?

Means that FASM decided to use imm8 with sign extention instead of using an imm of the size of the destination operand or using the AND acum, imm encoding.

Your first encoding corresponds to this encoding:
25 iw AND AX, imm16 AX AND i mm16

The second to this one:
83 /4 ib AND r/m16,imm8 r/m16 AND imm8 (sign-extended)

83D00F also represents (and ax,0fh)
^D instead of E

hurray for CISC !!!

There are a lot of redundant encodings in x86. Probably it's possible to have a good guess of which assembler was used to create the executable by carefully analising the encodings.