Chris, a good technique to trace errors like this is to simply use a pencil & paper and manually follow the instructions in your code. Set & change the values of your variables as you work through the code and your errors should become apparent. Later you will no doubt learn to use the debugger instead of pencil & paper.
There are numerous style issues (indenting, variable naming, layout, comments ...) that could be improved in your code, and 2 main errors.
The first error, as pointed out by ddanbe, is that you only need to calculate 1 average, not 10. The code he suggests will get around that problem. Then when you make that change to a single average you will need to change your comparisons and you may end up with something like this:
for i:=1 to 10 do if (average<pin[i]) then min_pl:=min_pl+1 else if (average>pin[i]) then max_pl:=max_pl+1;
This shows that your comparisons are the wrong way around, so your max_pl and min_pl values will be swapped. Better to use:
if (pin[i] < average) then min_pl:=min_pl+1 else if (pin[i] > average) then max_pl:=max_pl+1;