Because the IRS is nice for once and lets you deduct the contribution even though you don't contribute until March of the next year. That's so you can use your tax refund to help make the contribution. Nothing to do with distributions. You can't report a distribution you didn't actually take until the next year.
How did you report the distribution on your 2015 return without the 1099R form? They are tricky to enter even when you have the actual 1099R.
