The boring part is that the so called reasons are left unexplained.

haha alright. as much as i love 'the sound of my own voice', i just tend to feel a bit silly typing long and technical explanations that people often overlook let alone try to understand. but for you Jethro, i'd be happy to elaborate :P

the space before $1 is actually insignificant, it's the space before $2 that makes the difference. look what $str() produces here:

//tokenize 32 3 3 | echo -a $str(+$r(1, $2), $1)

as you can see, a string of +$r(1, <value of $2>) is generated. in the overall construction $calc( [ $str( ... ) ] ), the evaluation brackets force $str() to evaluate early, and the result is passed to $calc(). it is as if we used the following:

//echo -a $calc( +$r(1, 3)+$r(1, 3)+$r(1, 3) )

which, of course, works properly. but since we're interested in the general case, in which $2 could presumably take on any value, consider what happens if $2 is a larger number, say 1000. +$r(1, 1000) is 12 characters; the length limit imposed on the return value of any identifier is 4,150; thus the maximum value for $1 if we're rolling a 1000 sided dice is floor(4150 / 12) = 345.

in the alternative form, where the space before $2 is eliminated, look what $str() produces instead:

//tokenize 32 3 3 | echo -a $str(+$r(1,$2), $1)

$2 is left unevaluated. +$r(1,$2) is 9 characters, and so $1 can now be up to floor(4150 / 9) = 461 for $1 = 1000.

the increase in the permissible value of $1 is really a side issue (how often do you expect to roll over 300 dice?) - the main disadvantage that including the space has is that it allows code in $2 to be evaluated. for example:

//tokenize 32 3 $!finDfile(.,*,1,$+(echo,$chr(32),oops!)) | echo -a $!1- is $1- | echo -a $calc( [ $str(+$r(1, $2), $1) ] )

this happens because now, as before, it is as if this were used:

//echo -a $calc( +$r(1, $finDfile(.,*,1,$+(echo,$chr(32),oops!)))+$r(1, $finDfile(.,*,1,$+(echo,$chr(32),oops!)))+$r(1, $finDfile(.,*,1,$+(echo,$chr(32),oops!))) )

which makes it easier to see what actually happened. that's why i suggested removing the space - i'm quite conscious about providing code that i know to be exploitable :P