The RTP is calculated by taking the amount returned vs the amount staked. The games are designed to have a specific RTP% over a large number of game rounds (millions), therefore, an individual's RTP can range and be above or below this amount. .
The fewer the number of games played, the wider the normal variance of actual payback may be experienced.
To give an example, you could place 1x £1 spin and return £10, giving a RTP of 1000%. Likewise, you could place a £1 spin and return £0, giving a RTP of 0%.
If you have experienced a personal RTP% that is lower than the overall expected performance, this does not mean that the game is not performing within the expectation of the game. The overall expected performance is based on all pays and plays, therefore it is entirely possible for an individual player to have a personal RTP% below or above the expected performance.