It's......you guessed it, the Phantom Mech incident. Quoting from Sarna (http://www.sarna.net/wiki/Phantom_Mech),
"Kell's critically damaged 'Mech toppled to its knees right in front of Kurita's Warhammer who then fired all weapons at Kell's at pointblank range, but miraculously missed with each and every weapon although Kell did not even move..........Kurita fired all his weapons once more, only to completely miss Kell again."
So, there you have it; an in-lore explanation for lagshields! (and ECM Mechs.....since the phantom Mechs could not be locked on to.) And that's why mechs don't take damage when you slam them with everything; we've got phantom mech pilots running around! And PGI has the task of hunting down each and every one of them.....
Disclaimer: This is, of course, all a little tongue-in-cheek joke that is in no way serious. It just struck me that the uncanny ability of light Mechs to survive heavy fire bore quite a resemblance to this incident. XD
Edited by Scurry, 20 January 2013 - 12:53 AM.