I have personally found while hunting using park or field mode that using M2 or a single frequency like 20Hz, will give a more accurate vdi than using m1. I have seen m1 give VDI numbers that were inconsistent with what should be given. If you depend on the numbers, that will throw you off. I dig by general vdi numbers, tones and size of targets.
The depth and condition of the target will also cause the numbers to change, regardless of which single or SMF you use. For example, I dug a zincoln the other day that had 1/3 of it deteriorated and missing. It rang up very weird but strong. If I remember, it was like a 39 where a normal penny would have been 45/46(which is also the range of a dime). The solid hit on the tone and size of the target made me dig it.
You can tell the size of a target by swinging over it from several angles and look for the edge by tones. I knew it was small but the tone was solid. Curiosity got the best of me.