While I knew source deductions had to comply with CRA requirements, I didn't realize the method for lump-sum payments was so... overly simplistic (chalk it up to no experience doing payroll).
I'm surprised to find myself accusing CRA of approaching something too simply, but yes, they basically treat it like you're going to get that lumpsum payment every month for the entire year, in effect source deducting you at the highest marginal rates. So even if you make $30k a year, if you get a $10k lump sum it gets taxed as if you make $120k a year. That method disadvantages people as
@Weinie has pointed out, or if they just needed the extra money for something (anything... it's their money).
In any case, I'm always happy to learn something new.