He definitely looks like that because he was a white prime toy clone with a different trailer.
For those not yet knowing the origins, I think it was something like this...
It was originally the next incarnation of "Convoy"("Convoy" being the Japanese 'Diaclone' line's name for the toy of Prime) that was going to be known as something like 'super battle Convoy'.
Hasbro bought the rights to the mold and made it a completely different character from Prime altogether as they introduced it into their line.. Could they have been thinking of re-incarnating Prime like Galvatron, as Ultra Magnus at some point...?
So "Magnus" (The white prime) is the cab part, which on the toy was made from the same mold as Prime. It combines with the trailer to become the "Ultra Magnus" we all know by sight normally.
The idea would have been like a powermaster Prime, but 2 years or so before powermasters.
And the 1st pic put up of him earlier, is Magnus actually mid way through bailing out of the armour- something fans had be waiting a long time for. It was the first time in TF history more or less. And it was recent Dreamwave loveliness.