They seem to be increasing the principal by the interest earned during the "grace period".
(Not my definition of a "grace period". But apparently, it follows loan conventions, according to online sources [1].)
Based on that assumption, the principal would be the following for the corresponding grace periods and approximate payments:
I show the 0 grace period (row 1) to demonstrate the effect of approximating the payment.
Note that about 0.625% is added to the principal for each grace-period month. And note that 7.5%/12 is 0.625%.
Because only approximate payment are presented, we don't know whether simple or compounded interest is added to the principal. According to online sources [1], I would expect compounded interest. But look at the loan contract for details.
[1] http://www.investopedia.com/terms/g/grace_period.asp
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