Apple Numbers query

[GoSeigen]

I'm using iWork '09 but this situation may apply in later versions too.

In Numbers, when I add a row to the bottom of a table some of the formulas from the previous line get filled automatically to the new line. It seems to only be formulas but the thing is I can't work out what category of formulas, because some seem to propagate while others don't. An example of each:

Auto-filled to new row:
=VLOOKUP($ID 0023,$ID number:$Name,4,)

Not filled:
='M' 23+'U' 23

The above are in adjacent cells on the last row before the newly added one.


What exactly determines which formulas make it into the new row and which don't?

GS