Assuming you have large enough sample sizes, the MC result is actually the correct one here because it considers card removal from the folded ranges (UTG+1 to SB). This spot is NOT only dependent on UTG's range but also the players in between.
If the players after UTG call with more than 0% then their folding ranges will skew the card distribution, ie if it folds to the BB then there are likely more high cards than usual in the deck.
Set the calling ranges for UTG+1 to SB all to 0% and lock the opening UTG range, the result should then match the math mode.
(Just to avoid confusion, this was a reply to a deleted post
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