There are two scoring methods calculated by the EBU.
Masterpoints(MP) are gained by approximately the top 1/3rd of the players in each session. They are accumulated over the lifetime of a player and never decrease.
The National Grading Scheme (NGS) was introduced in 2012 to compliment the MP system. Unlike MP, a value in NGS denotes your approximate current performance. It is based on recent sessions (about 2000 boards over a maximum of the last 3 years). If a player achives a good result against the opposition, the NGS increases and vice versa. This means that a player's NGS can increse or decrease every time a session is sent to the EBU. It will take a few sessions for a player's NGS to become reasonably accurate.
Full details are available on the EBU web site.