Stock Information Tool (for free)
Do you know the difference between products in stock and products in your warehouse? The shop only knows the amount of products in stock, not in warehouse. So as soon as you've got unfinished (pending, on hold, processing) orders there will be a difference between those two stocks.
The .commercers Stock Information Tool will tell you exactly how many products are in stock and in how many are reserved by orders so there will be no problem checking the stock.
And best of all: This extension for Magento CE is completely FREE!