Tireminder TMPS diminishing accuracy

I've had my tireminder TPMS for about 2 years now. For the first year it was almost spot on PSI from what I would read on a good tire gauge. Recently, all 6 tires are showing 5- 8psi low. I checked all my tires cold over the weekend and they are exactly where they are supposed to be. I know 5 psi isn't a huge deal but why would it suddenly start showing larger ranges that are lower than the tires are? But if I was really 8 PSI low, I'd air it up to where it should be. But it's not low despite telling me that it is. I have changed batteries etc. Repeater is powered on. Every now and again I'll get an alert that I have a leak. My previous TST did the same thing (which is why I went with Tireminder). Their website says a 5 PSI variance is normal. Is this normal and is it worth looking at a better TPMS like Halo - although considerably more expensive? I guess if it alerts me before a blowout that's the biggest point, but I can't help continually asking - are my tires really low or is the TPMS wrong? Defeats part of the purpose of a TPMS.