It does fire down to 0.05, there's confusion in the spec sheet - Yihi said something much higher, but then Pioneer4You recommended a range of 0.05Ω to 0.10Ω. It definitely fires to 0.05Ω and says Low Resistance below that.
No I have the opposite problem - Set Resistance usually reads a little lower than my other mods/DMM ohms reader. Like a 0.12 will probably read 0.10Ω, at least when I'm doing Set Resistance. I haven't checked the normal on-screen reading carefully but assume it's the same. This is not uncommon I've heard.
First thing you can do is carefully clean the 510 with alcohol (99% IPA preferably, 70% is OK too). As well as a big bottle I also buy little swabs, that are sold as pre-injection swabs for nurses. Little pads soaked with 70% IPA. I take one of those and then with the head of a screwdriver or tweezers I push it into the 510 and then all around it, rotating it. Often I get a lot of black stuff when I clean a 510 for the first time in a while.
I also have a couple of spare toothbrushes which are great for cleaning threads. I spray a little 99% IPA on the bristles and then scrub round the 510 heads and gently on the pin.
That said a difference of 0.15Ω sounds pretty high, too high for dirt alone in most circumstances. it might be the wiring inside the 510. If you're technical it's very easy to get the cover off and inspect the inside of the 510 - maybe a wire is coming loose or the pin has sunk so that you're not getting a firm connection.
If you're not comfortable doing that, a return is your only option.
The resistance reading is vital for accurate TC and reading 0.15Ω too high will make TC widely inaccurate - like hundreds of degrees out. It should be usable for non-TC Kanthal still.
Here's a couple of photos I took of the inside, around the 510, in case that helps give an idea of what's in there.
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