にしては: an unexpected evaluation against a known condition
Separate the benchmark from a surprising result, and compare にしては with expectation-matching だけあって.
Set a benchmark, then depart from expectations
にしては gives an evaluation that differs from what a particular condition would lead one to expect. 初めて作ったにしては、よくできています。 means that it turned out well considering it was a first attempt, contrasting that condition with the good result.
The evaluation need not be positive. ベテランにしてはミスが多いです。 means that there are many mistakes for someone so experienced, exposing a gap between the expected standard and the actual performance.
Separate the benchmark from the evaluation
九月にしては暑いです。 means that it is hot for September. The speaker accepts September as the benchmark and evaluates the heat against it; this is not a general condition meaning that if it is September, it is hot.
Attach the expression directly to a noun, as in 初心者にしては, or to a plain-form verb, as in 初めて作ったにしては. Identifying the established condition first makes it easier to see why the following evaluation is surprising.
Compare the direction of expectation
経験が長いだけあって、説明が分かりやすいです。 means that, as expected from extensive experience, the explanation is easy to understand. The good result matches the expectation here, unlike the expectation gap expressed by にしては.
Saying 初心者にしては上手だ can praise someone while also implying low expectations of beginners. When addressing the person directly, consider how that premise differs from simply saying 上手ですね.
Does ベテランにしてはミスが多いです。 mean that many mistakes are natural because the person is experienced?
Show the answer and why
No. It says there are many mistakes despite the expectation that an experienced person would make fewer. にしては highlights the mismatch between a benchmark and the actual evaluation.