Blood pressure sitting vs lying down

I have been monitoring my blood pressure on a regular basis and always have higher readings when sitting up than lying down.  The diff varies by as much as 20 pts and in both arms.  I did have aheart attack in 1998 w/heart damage and now have 3 stents.  Last week had another blockage opened and now everything appears ok.  Any ideas why the readings would contiue to be high, 140's and 150's over 90 to 100 when sitting and near normal when lying down?  

[higgie]

easier to push blood side ways than uphill/downhill...this is normal