Since forever, the sideboard has been set to 15 cards.
I was actualy buidling sideboards for my decks, i play with friends, they play with my decks so i realy dont need to follow any rules anyway so ill set my sideboards to 20 cards....and let people switch cards...
Anyway, the point is, there is a limit of 4 copies of the same cards. So subsequently, if you want 4 of each, you end up having to do 4-4-4-3 in your sideboard....because 15 doesnt divide by 4.
Of course, sideboard doesnt require you to get 4 of each...usualy can be 2 of a card etc but even with 2, 15 doesnt work with 2 either.....
This argument has been going on for yonkers. All the way back to when sideboards were introduced.
Even if you set it to 16, some players might not do a 4-4-4-4 split but rather split their cards by 3, so on the 15 cards, you can do 3-3-3-3-3, but not so with 16.
It really doesn't matter, I've rarely seen a 4-4-4-3 split on sideboards. Sideboards accommodate the strategy of the deck and follows no particular pattern strictly on the number of cards allowed. I might have 2 ofs in my deck and 2 in my SB and swap out another 4 of to increase my strength against a known meta. In rare cases, I sometimes put in extra basic lands knowing I was going up against land destruction or land locks. Or swapped out cards strictly for colors, such as more blue cards to feed Force of Will.
If I argued for a 16th card, it would be to get more cards into my sideboard for better deck reconfiguration, not to ensure I can do a 4-4-4-4 split on my cards.