Not all fixed errors are added to the update immediately after fixing them. Improvements are added successively, in smaller amounts (the most serious and the most common first + minor ones, which have a small risk that something unexpected will happen).
Adding everything that is fixed right away is risky, because if any of the fixed errors has negative side effects, it will be harder to find the source of the problem, and the cumulative negative effects can be even more problematic 😕.
Therefore, new software updates are released more frequently but they are smaller (containing a smaller number of fixes) than one big update, with all the fixes included.
Also, many changes require prior testing, so if the error is not critical it may take some time to add it to the software.
That's why sending error reports is always useful and it helps developers to be aware of issues, even if all the improvements cannot be introduced at once 😉.