There can be several reasons.
Medicare generally bills in 3-month increments if you don’t have your premiums automatically deducted from Social Security.
Beneficiaries are responsible for multiple Medicare costs, including monthly premiums, deductibles, and coinsurance or copayments.
Most Medicare beneficiaries collect Social Security benefits, and for these enrollees, Medicare premiums are deducted from their monthly Social Security check. But if you haven’t retired yet, you have to pay your bill directly to Medicare.
Also, if you’re what Medicare terms a high earner, you may pay more for Medicare.
Did You Delay Signing Up for Medicare?
Your monthly premiums may be increased due to late enrollment penalties. These late enrollment penalties arise if you’re late signing up for Original Medicare (Medicare Parts A and B) and or Medicare Part D.
These penalties are added to your Medicare Premium Bill and could be why your first Medicare bill was higher than you expected. These penalties may be offset by enrolling in a Special Enrollment Period (SEP).