Ann: Lab Results Confirm Kimberlite Target Areas, page-175

I think its a combination of factors to why L259 has a low magnetic signature.
We know that the average size of diamonds has a lot to do with how hot the initial eruption was as the hotter it is the more diamond that gets burnt off on the way up.
Larger diamonds means cooler erupting kimberlite pipes.
I believe L259 erupted millions of years before the high magnetic pipes did and was a cooler pipe that ended up being covered by the surrounding pipes million of years latter.
IM more inclined to believe that the low magnetic signature is to do with it being one of the oldest pipes at lulo,and I think the best pipes of a low magnetic signature are buried by the younger pipes.
IM also quite certain the company mentioned something along the lines that the low magnetic signatures were to do with the pipes erupting much earlier than the high magnetic pipes.