Why may the quantity of products sent to a store be higher than the demand forecast?

There are actually a few reasons

Leftovers

Imagine an example of a product with 5 sizes (XS-XL). If the destination store only needs sizes S-XL, it will also receive units of size XS (even if it ends up with higher stock than the forecasted quantity). This is because it wouldn't make sense to leave the sending store with only the XS size. Doing this avoids any store having leftovers.

Forecasting is a probabilistic calculation

This means that, even if the forecast is for example, 4, it does not mean that we expect to sell exactly 4 units in the next X days (X being given by the chosen planning horizon). Instead, there is still a probability of selling the 5th unit, and the 6th unit, and the 7th, etc. even though these probabilities are much smaller.  

Therefore, the optimization algorithms will always try to send more than the forecast to make sure that we can cover part of those extra units that might be sold.

For more information check: Leftovers