There could be a few reasons for the testing differences, and this is sometimes a very tricky thing to figure out. But here are a few things to check.
1. Was the water that you checked from the same source in your house? Or was one test from the spa and another from the faucet? If one test was from the spa, was the water warmed up?
2. How old are the test strips that you had previously? They should have an expiration date on the bottom.
3. How long was the water sitting in your car before being tested by the dealer? Temperature can effect ph.
4. What kind of testing equipment did the dealer use?
If they use just a simple test strip test kit, I would probably find another retailer with better equipment for my testing needs.
All of these can effect the testing of water. But to answer your question about the water in the lines, it is very possible that the water from the tap could be very different from the 400 or so gallons going into the spa from your hose.
One thing you might want to try is take your water sample from the spa to the dealer immediatly, bring both sets of test strips and check the water at the same time as the dealer tests. This is the only real way to find out if there is a major discrepency. As a general rule, I don't like test strips and really only use them for a very general idea of a waters balance. Test strips from Wal-Mart do not have my confidence at all.
