If you have the patience to read there is a long post of explanation
A key issue is that share price return and fx impact are multiplicative factors, but for convenience the app has been programmed to show them as additive.
GAIN/LOSS as shown in the app
expresses the gain/loss in US dollars, converted to GBP at today’s exchange rate.
Tell me if you understand this point about the difference between multiplicative and additive effects. Or can better explain it!