Or HaChaim on Numbers 5:7/אור החיים על במדבר ה׳:ז׳

1 והשיב את אשמו, and he shall make restitution for his guilt, etc. The Torah again switched to use of the singular although the verse had commenced by speaking about sinners in the plural confessing their guilt. The reason is that when it comes to having to confess one's guilt all sinners are in a similar situation. When it comes to making restitution, however, this is not so as the Torah makes allowances for poor people whose sin-offering consists of lower priced birds or even a meal-offering only. We have learned this in Leviticus 5,7-5,13.