May sound like a silly question, but why does my disco roll away when I jack up one wheel?

I know the "e" brake is a trans brake, but what is different about these from say my old chevy 4x4 that won't roll away when in gear?
Thanks!

 

Not sure why it behaves this way, but my steps to jacking up my Disco is by putting the T-case in low range, engaging the parking brake, and putting the car in park. I also use wheel chocks to prevent the vehicle from moving.

 

@swissarmychainsaw You have problem, mine stays put with e-brake on and transmission in park. The e-brake is on the drive line, you will get a bit of movement but it should be minor. If you have a rare manual then 1st gear and e-brake on should do the same.