There are few reasons why we can have variance in the budget such as
Faulty figures in the budget due to errors made or duplication. You need to document it to avoid this mistake again in the future. Or assume that there is no overdraft but actual figures show that overdraft is necessary. You can go to bank and ask for one or try to improve your revenue stream / cash flow to stay within budget.
Next one is errors in the arithmetic of the actual results such as that we forget to charge people, forget about our costs or cash flows and etc. Crucial is the documentation to not happen again.
Reality is wrong – a natural disaster, strike or aliens can attack meaning the actual results are useless indicator. In the future, your budget should account allowance like this. Buying an insurance would be a good security
Lastly things like difference between budget assumptions and actual outcome can occur. Of course nobody is Nostradamus and knows future, but always look in the future and accommodate your budget. Focus on what we can do about it rather than spending time why the results are different.