The SALR is not a fixed number. Its value depends on the amount of water vapour in the air. Where the air is very dry it gets near to the DALR value. We all used to use the approximate temperate zone sea level value of 1.5, but the JAA changed the exam value to 1.8.
I don't know why they changed, but as it is an approximation the choice is not critical as long as we all use the same value.
Dick W