In giving the instructions for building the mishkan, sometimes the torah calls for "pure gold":
וְצִפִּיתָ אֹתוֹ זָהָב טָהוֹר, מִבַּיִת וּמִחוּץ תְּצַפֶּנּוּ; וְעָשִׂיתָ עָלָיו זֵר זָהָב, סָבִיב.
And thou shalt overlay it with pure gold, within and without shalt thou overlay it, and shalt make upon it a crown of gold round about. (Sh'mot 25:11)
Other times it just calls for gold:
וְעָשִׂיתָ בַדֵּי, עֲצֵי שִׁטִּים; וְצִפִּיתָ אֹתָם, זָהָב.
And thou shalt make staves of acacia-wood, and overlay them with gold. (Sh'mot 25:13)
Both of these are talking about overlaying wood, so it's not a difference between overlaying and casting. (I would expect 24k gold to be suboptimal for casting things that have to bear weight anyway.)
What is the significance of this difference? Is this a difference in language only and all the gold was actually the same grade, in which case why does the language vary? Or were different grades of gold used for different items, in which case why?
I haven't looked up all the uses of gold in the several chapters about the mishkan, but these are from early in Parshat T'rumah, where the instructions begin. I found nothing about this difference in Rashi or the Stone chumash.