New T-CELL-15 reading lower salt than 7 year old cell

I have a 7 year old Aqua Rite system with a T-CELL-15. I noticed this year that the salt level reading on the panel wasn't going up as high as expected when adding salt. For example, this spring after adding a lot of fill water, I added 200 lbs of salt which normally raises it about 900 to 1000 ppm on the Aqua Rite display (~25,000 gallon pool). It only raised it 700. Last week I added 80 lbs and it only raised it 200 ppm. I would have expected about 400.

So...I assumed my 7+ year old cell (in it's eighth season) was nearing the end of it's life. I purchased & installed a new cell and my salt reading actually went down 200 ppm (from 3000 to 2800). Instant salt reading is hovering at 2600 to 2700. It was 2900 to 3000 with the old cell. It's putting out 4.6 amps (it was around 4.8 before). Has anyone else experienced these numbers going down with a new cell? I was fully expecting them to go way up, and possibly needing to drain some water and refill to bring the salt level down. I don't have a salt test in my kit, so I don't know the true level, just what is on the display.

My unit has the 1.4 firmware level so there's no configuration settings for the cell. I replaced like for like with the original cell- Hayward T-CELL-15.

Here are my readings from this morning, before replacing the cell:

FC 4.0
CC 0.0
pH 7.5
TA 120
CYA 50 (I'm gradually bringing this up to 70 or 80)
Salt 3000 PPM (on the Aqua Rite display)
Temp 81

Has anyone ever seen the salt reading and amps go DOWN with a new cell? Also, I don't have a salt tester. Can anyone recommend a good one?

Thanks,
Steve

Well this morning, the salt reading was back up to 3000 PPM, though instant salt is showing 2700 (it was 2900 to 3000 with the old cell). I just realized that I do have a salt test after all. It came with my PS234 kit but I never used it before. The drop test read 3400 PPM, which seems to be in the ball park of the 3000 that the Aqua Rite display is now showing. So now I'm thinking it's probably just a minor variation between what the old cell and new cell is reading, and my old unit was fine all along. So I'll put the old one back on and consider the new one a spare!

Is there a shelf life for either chemical in the salt test (potassium chromate & silver nitrate)? The test kit is 7 years old. Neither reagent bottle was ever opened before today, if that makes a difference.

Thanks,
Steve