Wellcome to the wonderfull world of pantone (and color management)
One could think that pantone is sinonimus of a clear guide of color. In my opinion is not. The numbers has no sense. (Yea, I have read the meaning of the numbers... They have no sense :o) )
They change the cmyk and rgb equivalents from time to time. I collected some pdf references from over some years, not from a third party user, but from the oficial site, and the values were different.
I think you can not download them anymore from the pantone site I have not found an update for a cuple of years.
Ok some explanations
1) The standards of the fabrication of the inks (Swop, Eurocoated, Japan) Change. Lets imagine a manufacturer of inks find a new formula to make yellow that has no radioactive materials in it and it is purer and brighter.
After some time the ink is adopted as standard and as the yellow is brighter, you need a bit less of it on a formula to match a color. Then, the Pantone to CMYK values need to change.
2) The second explanation is from the standards itself. For example.
The Classical Swop V2 (american) standard assigned a maximum ink coverage to 300%. That was the maximum of ink in a photo in a deep black photo of the emptiness of the space.
But in other standards, like Fogra your ink can be 350%. Obviusly you need strech the values accordingly.
The explanation from the Lord of the Sith... Darth Vader, is correct, but you NEED to take into acount the cmyk profile you are using and the last version available.
So in this case I would not rely on the conversion of the rgb to pantone site, but in a well configured Ilustrator or Corel with the correct color profiles that you need where what part of the world you live.
Embeded profile or not in a RGB file
The RGB values are also modified in a file when adding (or discarding) a color profile.
In this case the values of a file without any color profile are simmilar to the ones Darth Posted, but the RGB values inside the aplication (Corel, Ilustrator, Photoshop) Will be diferent like (R76 G168 B195) + the color profile = (R0 G169 B198) (simmilar to the one posted by Darth Vader.
If you are using windows, you can use a small aplication to read onscreen colors like: http://www.color-picker.de There should be something similar for Mac.
One aditional note.
It is not a common practice to define a corporate color in the Uncoated chart, but in the Coated one. If you can define a new similar color in the coated guide, in my opinion its better.
This of course depends on the main printed usage. If you print more in a coated or uncoated paper. But it is easier to controll color on coated papers.