The fact that the drawing issue occurs in the IDE confirms it's a StringList StringGrid bug. Happens on my system too. also Win 7 Win 8, 64its. SVN Laz.
I'm looking at your code now, see if I can identify which property is the culprit, after that I'll check on MantraMantis, see if it's a known or not...
UPDATE: Definitely a drawing issue. The affected region is one scrollbar high. If I turn scrollbars off, or increase the width of the control by the width of a vertical scrollbar, then the issue goes away. Appears to have something to do with the widths of your columns being very close to the width of the control. Worth noting that at no time did I run the code, simply made changes in the IDE, closed the form, then reopened it.
Nothing comes up on a Bugtracker search of "TStringlist paint". Looks like a bug report should be raised.
UPDATE2: In terms of a short term fix to your problem, try On FormShow, set the StringList StringGrid width to 500, then back to 448. A hack, but works on my system. Not thoroughly tested :-)
UPDATE 3: Sigh. Nothing relevant appears to come up on a more relevant bugtracker search of "TStringGrid Paint" either