My ACTUAL mileage for the year was 21,397. Foolishly I took advice from a co-worker who said to round it to a lower number because the real mileage would draw attention!
The upside is that you can now get money back. You are just entitled to get money back if their is a correction in your favor.
This could be to your advantage if it is properly documented.
Respond with actual factual information with a notation of difference from the tax return.
Are they asking for information or have they proposed an adjustment?
If you document actual business miles with a log or other written documentation of actual mileage, then you should be able to resolve it to a no change audit or get a small refund.
If the IRS does not allow your additional miles, you can later file a 1040x amended return to claim your additional deduction and refund.
Do you have more than 1 client that is paying you a fee? Otherwise they may be looking to change your status to employee. It may be worthwhile to have a CPA review it before replying to IRS.