Win 10 Snap Assist - Inconsistent results between screens

I have a strange issue with Windows 10's snap assist. It's going to seem really pedantic but it's affecting a required workflow which I wont go into here.

I have 2 screens each with screen res 3840 x 2160.

The workflow requires 2 browser screens open and I use win 10's snap assist to place side-by-side.

On screen 2 snap assist results in two side-by-side browser windows each with a width of 767px which is just perfect for my purpose. The unmentioned websites render perfectly and are fully usable.

On screen 1 snap assist results in two side by side browser windows each with a width of 766px and this prevents the websites being fully usable.

To illustrate see attached screenshots from both screens where I have used Firefox's web dev tools and screen ruler to show what I mean. I'm just using google start page for illustration.

Images a & b - showing screens 1 and 2 in single window full screen browser mode - each with identical 1536px width

Image c - screen 2 operating correctly and as required in spilt screen mode using snap assist - 767px window widths each

Image d - screen 1 with snap assist 'discrepency' - 766px window widths each, small 2px between snapped windows

How can I get screen 1 to snap correctly to 767px widths eachand not lose 2 pixels?

To reiterate - this is not pedantic as it seems for my use case.
For anyone who hasn't lost the will to live reading this and might be able to advise me I'd be most grateful.