Why do some results have decimal points in?
This is because the computerised scoring has been set up to use Neuberg's formula. When a board is not played as many times as others, for whatever reason, the formula does a mathematical calculation reflecting the number of times it has been played. This gives a fairer result than the traditional method of reducing the top score. If there are 9.5 tables  and a Mitchell is played then some boards will be played 9 times and some 8. The boards played 9 times will have a "top" of 18, those played 8 times a top of 16. A pair that does well on the boards played 9 times will do better than pairs who score well on the boards played 8 times. The Neuberg formula avoids this anomaly. For more explanation see The Neuberg formula - English Bridge Union

How is duplicate scored and how does this affect strategy and tactics?
In match-pointed pairs (which is the correct term for the duplicate which is played in the club on most occasions) the making or defeating of the contract is no longer so important as it is at teams or rubber bridge.  The aim is simply to score more or lose less points than the other pairs holding your cards.  The amount you beat them by is irrelevant eg you may score 1360 and the other eight pairs with the same cards only scored 200 or you may score 210 and the other eight 200.
In both cases you have come top; the amount by which you win does not matter.  You receive 2 match points for each pair you beat and 1 for each you tie with.  In both cases above you will score 16 match points.  Each board is scored separately and each pair's total match points from all the boards is added up.  The pair with the highest percentage is the winner.

Each board is separate- if you have a very bad board, forget it, it is not -1000 points but only -4%.  This means that every board is important, even if you have no points at all, as you have to endeavour to prevent declarer making more than every one else with his cards.  It costs just as much to allow an overtrick in 2 clubs as to allow an overtrick in a small slam.   
You should try to set a target number of tricks for the contract.  For example if you can see that the majority of the pairs will play a hand in 4H and make it and you are in 3NT then you have to play for an overtrick to make 3NT+1 for 630.  If you go down it is no worse than 600 for just making 3NT since this will be a bottom anyway.
Be careful that you are sure that all the others will be in 4H.  
In defence you should aim for safety.  Try to avoid leads that will give declarer a trick he would not otherwise have scored eg do NOT lead away from an Ace or King, do NOT lead an unsupported Ace.

The difference between passed out and not played
A passed out board is scored as 0 to N/S. When this is converted to match points (see above), it may be a top (all the other N/S have a minus score), a bottom (all the other N/S have a positive score) or any result inbetween.

A board which is not played for whatever reason (out of time, misboarded etc) is deleted from the traveller for those particular N/S and E/W. The percentage for those pairs is calculated from the boards they have played so they have a lower total than other pairs but not necessarily a lower percentage. Example:
     5 boards are played with a top of 10 match points so the possible total is 50.
     Pair A score 28 out of their possible 50 so their percentage is 28/50 x 100 = 56%
     Pair B have one board not played so have only played 4 boards. They score 23 out of a possible 40 so their percentage is 23/40 x 100 = 57.5%
Please be careful to put the right description on the traveller as the two situations are different and have different results.