Update:
I asked them for an explanation of calculation and it's because of the first three months being 90 days (in my case 15/12 to 15/03 31 + 31 + 28) and the next three is 92.
I worked it out as follows and it is pretty accurate.
Take the oustanding amount on a given date when you have paid mortgage. i.e. 15 Dec.
Multiply by interest rate say .04 to get the interest due.
Divide by 365 to get daily rate and multiply by how many days till your next monthly pay date.
Repeat the above using the new figure.
Example
15th Dec - 100,163.59 @ 4.4% = 4407.20/365 = 12.075 per day x 31 = 374.31
15th Jan - 99,334.12 @ 4.4% = 4370.70/365 = 11.975 per day x 31 = 371.225
15th Feb - 98,504.65@ 4.4% = 4334.20/365 = 11.87.45 per day x 28 = 332.4866
15th March interest due 1078.02
15th Mar - 98,753.48 @ 4.4% = 4345.15/365 = 11.905 per day x 31 = 369.055
15th Apr - 97,924.01 @ 4.4% = 4308.66/365 = 11.8045 per day x30 = 354.135
15th May - 97,094.54 @4.4% = 4272.159/365 = 11.7045 per day x 31 = 362.84
15th June interest due 1086.03