I don't know what sensor on the phone it uses but it's on the screen side and it's very angle sensitive, which could be because of screen protector or something.
on the screen in the app, theres those 4 little corner lines, if you dont get the light source entirely within those 4 square lines your going to under read on PAR and luminosity both.
Your reading there, getting 120 ppfd, is in reality, probably more like 200 ppfd fwiw. Your light source, if outside of those 4 corner lines on the screen, is not hitting the lens directly, its coming in at an angle.
This even screws up some grow light companies when they take measurements with legitimate PAR meters at the side of tents for their lights. They point the sensor straight up, which is actually away from the light source (which is in center of the tent used for testing). The grow light companies that know what's up use the other company's ignorance hard to their own advantage too, and make it look like their lights somehow cast a better foot print from the same lenses in the same wattage class and same form factor lol. Some light companies that will remain un-named even use manipulation of this information to allow themselves to get away with lower output and fewer diodes/wattage for the same class of lights because they know how to take more accurate readings in a tent, and their more accurate readings tend to make it seem like their lights cast a better footprint for less wattage, implying efficiency increase etc. When in reality you actually are getting less output, on the same footprint compared to other panel lights.
This is entirely why generic unbranded panels using the same drivers and diodes seem like they arent as good when you look at the par maps, even though they should be based on descriptions and spec listings, its all because those arent actually made by ddicated grow light companies, and the people making them dont actually know how to take accurate par readings lmao.
im about to pull a Lb or so from four under-volted $25 panels in a 3x3 that said 600ppfd at 10" in center of a 3x3, but said 45ppfd at the edge at 24" xD...... no ones buying them, i watched em drop from 55 to 25 over the course of a year, then bought 6 lmfao. Currently running 4 from a discrete switch mode power supply. They probably would have sold if the people taking the light measurements actually knew how to do it lol.
I can mimic the 45ppfd 24" result with these panels, if i use a phone app, and point directly straight up at the roof of the tent with one at 100% at 24".... but if i angle the lense to source, it jumps to like 180 ppfd, at what is essentially 2.5ft away from a single panel off-axis. that's actually pretty damn good for a single 55w panel. better than average actually.
All digital lenses are highly angle sensitive, even a high end lens for a spectrometer.