score:8
Your computation appears to be correct, as confirmed by the Schengen calculator provided by the EU Commission. This may be a simple mistake because a maximum length of stay of 89 days is somewhat confusing (the calculator is based on the visa-free rule of 90 days).
For the sake of completeness, note that there is no requirement that the period of validity and maximum duration of stay coincide. For example, a visa could be valid for 10 days between February 28 and May 28 and each condition has to be evaluated separately. But in this case, there really are 89 days between February 28 and May 27, including both those days.