Inconsistent Readings

Two weeks ago, in preparation for spring start-up, I reached under my mesh cover, and took a sample using last year's K-2005 test kit.
Values were
FC-0
TC-0
pH-7.2
TA-0
CYA-0
CH-20

I was expecting TA, CYA, and CH to be about 25% lower than last years closing due to melted snow. So I didn't believe the TA, CYA, or CH readings. So I measured three times re-read the measuring instructions and got the same results.

My first guess was that the reagants had gone bad, and I'd have to pick up some more. I didn't get around to it, but when when I was in the hardware store I picked up some test strips to confirm if my tests were off. The test strips provided similiar results.

Today, I removed the cover, connected the pump and started cleaning up the water. Still hadn't picked up the reagants so I thought I'd have to go to pool store to get sample measured. But first I tried my own measuremnts using last years test kit.

Cl-0
pH-7.2
TA-100
CH-250
CYA-40
Test strips measurement was reasonably close to same values.
The water tests for both the K2005, and the strips changed significantly. So the issue is not my test kit reagants:confused: .


Question #1
Any idea why the difference between measurements 2 weeks ago and today?

I have two guesses, but neither seem plausible to me.

My first guess is tests provide accurate results above a specific water temperature. First reading performed at water temperature around 45F, today water temperature was 55F.

My second guess is that the pump/water circulation had an affect. Is the CYA or TA low at the water surface after a long winter. If so, vacuuming would have circulated the water around to be more uniform?


Question #2
Do I need to replace all the reagants every year or just some?
__________________
Carl Schnurr
96,000 litre, vinyl liner, 1.5 HP 1speed pump, sand filter, SWG, installed 2005.