Direto power in Zwift

I’ve been using my Direto in Zwift for a few weeks now and I also have a power2max crank that I run alongside the Direto. At the moment Zwift power is from the Direto and I record the power2max on a Garmin. Unfortunately I normally see a 15-20W difference in power, with the Direto/Zwift combo always higher when using simulation mode. If I use Erg mode though, the power recorded by Direto/Zwift is a very close match to the P2M/Garmin. I’ve done the ‘calibration’ for the Direto using the Elite app - all fine there. Power numbers compared using DCR Rainmaker analyzer tool.

So I contacted Elite support and queried the discrepancy and part of the reply states:

“On Zwift the power isn’t measured, it is calculated from the speed value sending from the direto and in different type if you are on simluation or in erg mode”

I don’t get this at all. As I understand it, the Direto is measuring power, yet Zwift apparently isn’t using this value. Instead it takes speed and then calculates power from that and apparently does a different calculation depending on if its using Erg mode or not. And only when using Erg mode does it seem to get it right (assuming the P2M is considered accurate).

I certainly don’t expect identical numbers from the Direto and P2M, but I do expect some consistency in the numbers within Zwift. One option is for me to use the P2M as the power source, but that’s a pain because I’m using the Android app with bluetooth and my P2M is ANT+. I have a NPE cable though so it is possible, but I’d like to understand what Zwift is/isn’t doing before I use the P2M.

Any insight into how Zwift uses info from the Direto very much appreciated.

And to add to this - when I use the Elite app and compare power readings to my power2max, the figures are near identical. So Zwift is obviously doing something other than just using the power readings from the Direto (apart from in Erg mode where it may be using these).