1. Don't go by air tests...you can get a baseline but there are variables like where you test.
If you did this test in or just outside your home EMI interference like Wi-Fi and other things can affect depth.
2. Yea...hollow gold will react differently than solid gold.
3. The Compadre with the 8" coil can find the coins you tested a bit deeper than that...sometimes.
Not all detectors are calibrated exactly the same in the factory...you might think they are but nope.
It is pretty well known that buying Tesoros new you have an equal chance of getting one of three kinds...weak, medium or normal and there are some that are extra hot.
Most are medium and yours is close, from what I have read most will act like yours or go about an inch further or so out of the box with that 8" coil.
Some owners got further than that.
There is an adjustment inside that will enable you to turn up the sense a bit higher and maybe get a bit more depth and it is pretty easy but you can seriously screw up your detector if you don't know what you are doing.
My 7" coil Compadre had this adjustment and I am getting close to your results with that smaller coil.
What to do...
First take it out and hunt with it in real world conditions.
You might be getting further than you think out there in the open away from high EMI and you might be worried about nothing.
Second call Tesoro and talk to them about it.
Yea, they are having some issues right now but the repair people are still there and might offer some help.
Who knows, they might want you to send it in so they can recalibrate it for you if they think there is an issue but yours seems to be acting pretty normally IMO.
Third, you can turn up the sense, there are tons of info on how to do that and what to adjust all over the internet.
Fourth, 95% of the treasure we look for is usually at about 5-6" or less in depth, most of mine has been, anyway, and you are hitting that region.
If you are expecting to consistently find tons of targets way further I got news for you...you didn't buy the right tool for that task.
My Compadre cost me $130 used and I am the third owner, minimum.
Mine pretty much hits the depth yours does, I have never worried about it, and I have found over 10X's what it cost me in coins, silver and gold over the years using it part time.
It has provided me with hours upon hours of thrills, fun and joy, there is still nothing on the market even today that can match it for what it can do vs what it cost pound per pound.
It finds coins easily, can find targets laying closer to big metal than most any other detector out there and it has the ability to sniff out all chains and jewelry including silver and gold like it is a religion with this thing.
Quit worrying about it...just go out and hunt.
https://metaldetectingforum.com/showthread.php?t=87892