Difference between . access and [] access

hi all
i refer to this tutorial http://www.kirupa.com/developer/mx/mousetrailer.htm
what i found strange is that i can’t use . to refer to movie clip instance in this tutorial, it only works if i use [] access
for example:
“_root.instancename” versus “_root[instancename]”
only the second one works…
please examine the short code i attached just cut and paste the code…play the movie…and see the difference,
anybody can explain why? or i did any mistake?
thx for help

attachment below