It's not that sweepstakes law requires that entrants answer a math question exactly. It's that for-profit gambling - spending money in exchange for a random chance at profit - is illegal outside certain limited contexts (charity events, licensed casinos, and provincial lotteries), so sweepstakes incorporate skill-testing questions so they can legally claim to be games of skill rather than chance. Businesses came up with the skill-testing question as a loophole, and the courts have resigned themselves to allowing it, though they've made determinations as to minimum difficulty of the math question to qualify. It's essentially the same reason why every "find this thing in something you buy and win a prize" contest in the US says in the fine print that you can also enter by mailing in a 3x5 card instead, and has to declare in any promotional material "purchase not necessary for entrance" or something along those lines. That way they can legally claim not to be for-profit gambling either; you can enter for free, so it's not gambling. (Which is also a work-around done in Canada sometimes for sweepstakes instead, but the skill-testing question's more popular there; probably because it's a lot less work for companies, I'd guess.)
Well, the alternative is either making sweepstakes illegal, or trying to write a law that would make sweepstakes legal while not opening the door for the sort of gambling the law was supposed to stop. It's a near-impossible needle to thread without introducing even worse loopholes, so they took the path of least resistance. Edit: There's some more detail on the legal history of the whole thing on this page. For what it's worth, the case law that led to people using skill-testing questions to get around it was the Canada Trust Company using this question: multiply 228 by 21; add 10,824; divide by 12; and subtract 1,121. Much more difficult than the modern ones, but there's likely no interest in prosecuting modern harmless violations.
Most legal interpretation does rely on the idea that a common-sense reading should rule, which is what prevents most loopholes from actually working in practice. (Most of the time, loopholes only exist when someone can make a legitimate and believable argument that a common-sense reading would provide for the loophole, usually because of an error in drafting the law or a misreading of the text on the part of the legislature.) In this case, though, it's a combination of the difficulty in drafting, and of the government simply not caring to prosecute because the law isn't meant to bar sweepstakes in the first place, and so that bare minimum of effort is good enough for them. They don't care that this is something that probably wouldn't hold up for the sweepstakes in court, because that's not why the law's there. But if, say, someone tried to set up a keno parlor with exactly the same skill-testing questions to the winner, then you can be sure they'd come down on that. (Which would probably then ruin it for sweepstakes, which would then have to find an alternative, yeah. )