Why has the GBP value of my Tally account not increased by the GBP amount of my deposit?

When you deposit fiat currency (e.g. pounds) into your Tally account, you are exchanging fiat currency for Tally. As with all foreign exchange, there is a buy price and a sell price. The difference between the buy price and sell price is known as the spread, which accounts for any disparity between fiat funds transferred into your Tally account and the fiat value shown alongside your Tally balance. Tally does not add any margins or fees to the global gold wholesale price we receive.