why use ”;” but not use ”,” ?

I think this may have been an overkill.

The sentence would have been fine with a "," and a "and", or a ";" without the "and". Your tutor might have been contemplating one or the other, and accidentally used the ";" and the "and" combination, which in my opinion, is incorrect.

Without the conjunction "and", there are really two independent sentences, but the two thoughts are so closely related, that separating them with a semi-colon (;) is acceptable.

However, the flow of data from one year to the next with the same calculation formula also qualifies the two calculation as one single thought, so joining the two parts together with a conjunction (and), and separating the two parts with a comma (,) is also acceptable.

Since this may affect your confidence in your tutor, I would really like to hear from others. Also, you may want to ask your tutor directly. English is a fairly complicated language; it is very easy for us to occasionally overlook certain aspect of it.

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祝您好運!

Agree totally with 001.

Please ask your tutor and share with us.

Thank you.