HP50G and built-in variable sigmaDAT

[Gil]

If I want to calculate the built-in CORR function between two variables, the first one situated in the first column and the second one in the second column of the special sigmaDAT variable (in the HP50G),
I cannot use the (apparently logical) exact mode
(1 2 0. 0. LINFIT )
'SigmaDAT' STO
in order to execute afterwards
the built-in command-function CORR.

Instead, I do have to use the approximate mode
{1. 1. 0. 0. LINFIT } (dots inside the {}).
'SigmaDAT' STO
and then execute
the built-in command-function CORR.

Why this (seemingly illogical?) restriction?

I thank you for your clues relative to that matter.

Regards,
Gil