VUAG (and other stocks) price difference, why?

I’ve noticed this before, why are the stock prices on Freetrade sometimes not the same as on other platforms. I realise it would be difficult to make an accurate comparison during the trading day, but surely the closing price should be the same on all platforms?

An example from today is VUAG (Vanguard S&P 500) which in my Freetrade app is shown right now as £54.85, but in iWeb (where I have an ISA) and on Yahoo Finance it is shown as £54.90.

Looks like FT and Google Finance are showing the Last Trade Price of 54.85.
LSE is displaying 54.91 as the close price even though the trading High was 54.85.
I guess this is probably the difference, I’m sure someone has a better answer

the Bid +Ask/2 which is 54.91

It has something to do with the bid/ask price and freetrade use the median. Correct me if I’m wrong.