nreavaille, I can’t make excuses for the drawbacks in the CV testing system. Clearly, your combined 600 meters dropped by 18 seconds. But, it’s just the nature of that test to not move the needle much.
The 1,000 meter/yard option is a better choice for some athletes. Even if you had performed that test, let’s say you dropped 20 seconds from your 1,000 test (18 seconds over 600 = 20 seconds over 1,000 as an estimate), that would only be a 2 second difference in your threshold pace anyway. 18 seconds feels like a huge drop in time, but it’s really not over the full 600. Using either system, your zones are really close to being the same.
But, you should celebrate the improvement anyway! That’s still a sign of improved swim fitness.