This is best explained with an example. Let's say you carried out these 3 transactions:
Bought 2 BTC in January
Bought 3 more BTC in Feb
Sold 4 BTC in March
Here, even though you only had 1 sale transaction, you would see 2 disposals on your tax report. This has to do with the way your costs are calculated (FIFO, LIFO etc).