Does any one knows why ACR/LR gives a different WB temp from the one that is setup on the camera?

Here is a list of the shift

5D2 ACR

2500K 2550K
3000K 3000K
3500K 3450K
4000K 3850K
4500K 4300K
... ...
5500K 5100K
6500K 5900K
... ...
10000K 8300K

It happens with other canon cameras as well.

Yep. It does this on my 1DsIII as well. Mine also displays a tint shift away from zero. I think ACR reads the metadata parameters you selected and then interpolates that into its own color recipe, displaying how its numbers jibe with the color temp number you selected in the camera. It's gotta read Canon's code, then spit out its own version of that, which looks like it may not be the same numbers.
At least, I think that's how that works...

It sounds counter-intuitive, but the color temperature and tint are in the praxis not absolute numbers, because the illumination is not from an ideal black body. They are to be interpreted in relation to the correlated color temperature. Different raw converters may use different refernce points and calculate different temperature and tint, while yielding the same (or close) result.