Array issue

[Michael Lew]

Dear listers

I have struck an unexpected issue with an array. I assumed that the two
following code snipets would be equivalent, but they are not.

Version 1 
    put random(10)-1 into tnum
    add 1 to iHisto[tnum]
Version 2
    add 1 to iHisto[(random(10)-1)]

Version 1 does what I expected, and, I guess is more readable and so
preferable for that reason. However, I can't see why the two versions differ
in their effects.

Here is the full code of my button for anyone who wishes to try it out (the
different versions are indicated with comments):

on mouseUp pMouseBtnNo
    put 10000 into oreps
    put 100 into ireps
    set the cursor to busy
    repeat oreps
        put 0 into kount
        repeat ireps
            add 1 to kount
            -- **next two lines
            -- put random(10)-1 into tnum
            -- add 1 to iHisto[tnum]
            -- **OR the next one line
            add 1 to iHisto[(random(10)-1)]
            --
        end repeat --inner
        combine iHisto by return and comma
        sort lines of iHisto numeric descending by item 2 of each
        add 1 to ohisto[item 2 of line 1 of iHisto]
        put empty into iHisto
    end repeat --outer
    combine oHisto with return and tab
    sort lines of oHisto numeric ascending by word 1 of each
    put oHisto into fld "OutputFld"
end mouseUp

Thanks,
Michael Lew