I think it's because the ball did not technically cross the plane of first/third base in fair territory before ending in foul territory (by definition when the ball hits the base, it has crossed the front of the base in fair territory). Bizarre situation, but it's the only sensible explanation I can come up with.
No only sensible, but supported by rule.
The pitching plate is in front of an imaginary line running from the front corner of 1st and 3ed bases. The fact that it hits the plate means nothing.
This is one of those plays where you could win a few beverages of your choice with friends and family buying.