Well, here's some quick calculations.

On 3D6 there are a total of 216 possible combinations of the dice. Only 20 of these will result in a 15-18 (penetrating). That's 9.26% of the time. Out of these 9.26% penetrations, only half will result in a kill of the Monolith. Thus we have a 4.63% chance of a kill through penetration.

The chance of rolling a 14 (glancing) on 3D6 is 15 out of 216, or 6.94%. Out of these 6.94% glances, only 1 in 6 will kill. Thus we have a 1.16% chance of a kill through glancing.

So we have a hit at 83.3% (BS 5).

A hit results in a penetration kill 4.63% of the time --> 3.86% chance of penetrating kill
A hit results in a glance kill 1.16% of the time --> less than 1% chance of glancing kill.

Sounds like a far cry from 37%.