Yer either hallucinatin' or the victim of some fakes. 
Ultima I & II do exist (seek and ye shall find...) for the Amiga, a quick read of the docs explains that they were in fact written and released as PD by someone who wished to complete the Ultima series on the Amiga, so therefore Ultima I & II for the Amiga do exist, even though they may not have been written by the same company who released the others...
It even mentions in the docs the fact that these were not written by the original developer and were written quite simply by someone who was an Ultima fan and wished to make available the complete Ultima series to Amiga fans. The creator of I & II even goes on to say "sue me" if the original creator of the Ultima series was not happy with them and as far as I recall, no-one was ever sued, so it seems everyone lived happily ever after... the end.
