Question about Interest Payments on FRM

[Max Chase]

Purchasing a commercial property with a fixed rate, 5 year term loan, 20-year AM. On the AM schedule the interest payments hop up and down, the principal payments due too, while the total amount paid per month remains the same. I have only ever seen the principal getting higher and interest getting lower on an amortizing fixed rate mortgage. Any clues before I circle back to the lender?