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I don't wonder why the bible isn't understood any more than it is! As long as people read the bible looking for grammatical errors, comprehension will go lacking.
The topic in question is simply a matter of penmanship or language translation. There's no way the writer meant that Jesus sat on both animals! Why spend bible study wondering about something like that? If something like that commanded two seconds of my attention, I'd wonder why!
In my opinion, the composition of this question matches the confusion in the composition of the scripture that is cited...