What is the story behind the strange Tennis scoring pattern?

It was only during the recent French Open (2012), I got a little curious about the unique scoring pattern in Tennis. The reason seems to be a theory but nevertheless worth knowing for all the Tennis lovers.

Couple of those popluar theories are:

1. A clock face might have been used to show the score. The clock hand was moved a quarter every time a player scored a point i.e. 0 to 15, 15 to 30 etc. But as we all know the scoring pattern is 0,15,30,40 and Game. It does NOT include a 45. The reasoning is based on the premise that in Tennis a player needs to have a difference of atleast 2 points to win a Game. Hence a score of 40 might have been chosen instead of 45. For example when both players score 40-40, the next point will take a player to 'Advantage' (50, if we can call so) and the next point to 60 and the clock cycle completes.
A '40-50-60' sequence seems easier to handle than a '45-52.5-60' or a '45-50-60'

2. Scoring system was inherited from its predecessor game. A popular game in France where players used their hand instead of a Racquet. The length of the court was 45 feet on each side. On winning a point, the player had to move forward by 15 feet, another 15 feet  for the second point and 10 feet for the third.

There is another aspect which baffled me for a long time. Some times we see Set scores like 70-68 (Isner Vs. Mahut, Wimbledon 2010) and most of the other times the Set ends with scores like 7-5 or a 7-6 (7-5). How is the winner of a Set decided? 

The answer to this is that in few matches there is a mandatory tie-breaker once a Set score reaches 6-6. In other matches, the Set (called as Advantage Set) continues till the difference in number of games won is 2, like in Isner-Mahut Game. It is based on the fact that a player who serves has an advantage over other player and hence to win a Set, a player should break the service atleast once.

References:

Very well articulated in  http://en.wikipedia.org/wiki/Tennis_score