MSDN:

All print providers must export the initialization function, InitializePrintProvidor. Pointers to all the other functions must be supplied in a PRINTPROVIDOR structure. (Note that these two names are misspelled, but are consistent with the names that appear in the header file, winsplp.h.)

Of course, one can’t correct the spelling of one’s functions, else one breaks the principal of backwards-compatibility. (via Raymond)