First of all, it is not always a reliable value
We know that the health of the battery indicates the level of deterioration of the same, being 100% its best condition and 80% a level of wear that already indicates that it is advisable to change it. However, the exact procedure that iOS uses to calculate this percentage is unknown, beyond the fact that it is based on calculations from various algorithms. Precisely the complexity of these operations could be enough to determine that it does not always show the real percentage.
Proof of this is that there are times when after restoring the iPhone or updating its software we find completely different percentages, sometimes even higher. This occurs because they are recalculated, so this is also a reason why you may have seen that it has dropped suddenly.
Causes that justify a decrease in the percentage
We start from the basis that the battery is a component that will inevitably deteriorate over time. Knowing this, we already intuit that the percentage will end up falling sooner or later, although it is true that there are certain actions that favor the deterioration to occur in less time.
The use of non original chargers, whether cable or charging base, is one of these factors. Not being prepared for optimized uploads, they can cause the iPhone get overheated and therefore there is further deterioration. In the same way that this happens if the device is charging intermittently, connecting and disconnecting it continuously.
Although if there is a factor that above all affects battery health, it is to use fast charging adapters frequently. Apple limits the fast charging power in its devices, reaching at most 20w in more recent models, this being a lower capacity than many other terminals, but it also affects. Nothing happens to charge the device with an adapter of this type from time to time, but if it is done constantly it will end up being noticed in the degradation of the battery.
And although to a lesser extent it influences, also the fact that consume full charge cycles, that is to say from 0% to 100% (even in a fractional way), it causes the battery to suffer more wear in the end. In any case, our advice is not to be overly obsessed with this parameter and that if you suspect that the component is faulty, go to Apple and request assistance to see if your iPhone needs a battery replacement.