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Because it is wrong in ways they have learned to expect. It shows people at full capacity who are on leave, counts part-timers as full-time, and has no idea about the two days a week somebody spends on internal work.
So delivery keeps its own version, which is also wrong, and the two are never reconciled.
Three things have to be accurate before anyone will trust it: working patterns including part-time and compressed hours, approved leave, and the standing internal commitments that are not project work.
The third is the one most often missing, and it is usually the reason the numbers feel wrong to the people being counted.
Yes, deducted, or the number is fiction.
A percentage over a period, per role.
Yes, on approval — availability crosses, reasons do not.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.