On our internal mailing list a user asked "When I retrieve a column of type JET_coltypBit that is set to true, why do I get 0xFF and not 0x1"? 
My co-worker Ketan had to reach deep into his memory for the answer :-)
In the very old days, the Jet API was designed as a replacement for the BC7 ISAM. BC = Microsoft Basic Compiler.
"When Visual Basic converts Boolean values to numeric types, False becomes 0 and True becomes -1."
Now why would do they do this?  Because Basic treated Boolean/Bitwise as being the "same" operation. What does "NOT" do? It flips all the bits; hence
-1 = ~0