Using concatenation operator (&) before ROUND and SUM function in Excel

I'm using Microsoft 365's Excel on Windows 7. I'm trying to understand why this formula:

="S/ "&ROUND(SUM(E39:E40),2)

Will not provide an output to 2 decimal places?My intention is to concatenate "S/", the local currency, to the numerical string. The range of cells used are formatted as number to 2 d.p.

I can achieve the intended result by formatting the output cell to the currency in question. But why won't it work using ampersand?

Any clarification most welcome.