What care do reviewers take when writing a review for a published paper in mathematics?
MathSciNet reviews are not the kind of reviews an editor would get for a submitted paper. Rather, what is posted on MathSciNet is more of a summary of the article and someone putting it into the context of the broader field -- something that would help a reader understand not only what the paper provides, but why it is relevant. As such, the authors of these reviewers don't have to go through all proofs with care -- they don't even have to understand the proofs as long as they understand the theorems.

On the other hand, reviewers of math papers often go to great lengths to ascertain that the proofs in the paper in question are in fact correct. That typically requires a great deal of care, detailed insight, and work.