My guess is that when you began your subscription, you immediately got the current database which would have been valid before your subscription date. If they had done it the way you're thinking, you would have had to wait until the beginning of the next update to get the database, but that clearly wouldn't make sense, so they technically start your subscription one period earlier than your date...
RV-8 #81077 Super Slow Build
Titan IOX-370, Dual P-Mags, AFP FM200A FI, Sensenich GA Prop
First Flight November 20, 2016 - Phase One Complete June 1, 2017