Net Billings totals and Revenue Recognition totals for the same time period will not typically match because they track revenue differently.
The Billings report will simply report on the number of money that came in the door from any invoice successfully processed (or money out the door from refunds). For example, if a subscription charge went through on May 1st, Billings will report that money as having come in on May 1st.
Revenue Recognition works differently if you select "Evenly" as your recognition method. This means that revenue for a charge is recognized evenly every day over the range of time specified.
For example, let's say you have a monthly subscription that charges a recurring fee of $99/month and a customer is charged on May 1st, for a billing period of May 1 - June 1. Revenue will be recognized over the period billed, in this case May 1- June 1. A portion of the revenue will be recognized each day, so revenue will appear in the export for both May and June on this charge. Since $99 divided by 30 (for a 30 day monthly period) equals $3.30, this is how much would contribute to revenue every day. Therefore, if you pull the Revenue Recognition export for May 1 - May 13, you will not see a total of $99 for this subscription. You would see $3.30 contributed for each of those 13 days. $3.30 x 13 = $42.90, so this is the total revenue recognized you would see for the time period of May 1 - May 13 for a $99 subscription charge. The rest of that charge would appear in the days of May 14 - June 1.
You can see how that total ($42.90) is significantly less than what Billings would report for that same charge. Since Billings reports that charge on the same day it went through, you would see a total of $99 contribute toward the Billings total for May 1st whereas Revenue Recognition method of Evenly spreads it apart over the billing period.