AHI 0 after being 16+

Hello,

I just made this profile for my mom, which is 79 years old. For the last 10 days we are trying to find a suitable cpap equipment for her.
On the initial tests she had 64 AHI and depending on the different cpap devices we tried, her AHI decreased like:
1sth day - 21
2nd day - 29
3rd day - 18
... between 16 and 20
9th day - 16
All the tests were made with ResMed AirSense 10 AutoSet, with full face mask. There were almost NO leaks and with Oscar software I found out that among other apneas/hypopneas there were like 20 apneas 40-50 seconds each and 95% of the time the preassure was 15 (mm water I think).
A day before yesterday we got a brand new DeVilbiss IntelliPAP AutoAdjust Smartflex and a NASAL mask.
Second day in a row the AHI is 0 (zero). The 95% preassure is 3.5. She sleeps more than before and says that she feels fine.
My question is: Is that even possible? Maybe the new cpap machine is broken and not detecting apneas?

Thank you!

P.P. The new cpap works with some codes which I post at intellipap site, but they are not that informative as the Resmed + Oscar were...