If you do the following on your financial calculator,
10000 [FV]
9850 [+/-] [PV]
0 [PMT]
91/365 [N]
[CPT] [I/Y]
you come up with i = 6.32%. Which is greater than our lecture notes calculation of 6.18%. Why? Because you instructed you calculator to annualize i by compounding every 91 days. The calculator solved the equation:

While the method above makes sense and is a legitimate measure of an interest rate, the method in the lecture notes, known as a bond equivalent basis, is what we use by tradition in financial markets.