Can someone please explain why I lost 7 rating points after winning a game ?

I was playing a player with a rating of 1008. I have 1497. I won the game with him but he went to a rating of 1007 and I fell to 1490. Why is this ?

The White Knight.

I have just discovered that this is possible for "p" players. I don't understand it but it would seem to be expected.

p = provisional.

Until you've played 20 games, you have a provisional rating and it will change seemingly at random (including going down after a win) in an attempt to rapidly stabilize.

Don't worry about it - play some more games, hit the magic "20" mark and you'll return to "win = gain, lose = lose" ratings changes.