In the FIFO methodology, price assigned to shares is the price at which the shares are bought. This is assuming that the first shares to be sold will be the first ones that had been bought.
In weighted average cost methodology, the average cost is calculated as the total purchase value of a holding, divided by the number of units purchased.
Both methodologies use the same portfolio holdings, but the unrealised profit is different. You may see such changes within your own portfolio due to this change.