As a probably older fart, I recall that when I was in a company medical scheme the cost was over £200 a month.
Have you found out what the company was paying for your insurance? You wouldn't have got a P11D for benefits you received in 2025/26 tax year yet, so you might have to ask the company or guess a larger number from your previous P11D.
My guess from your info is that the cost to the company was £148 per month, and they put this on your payslip as a notional income so that the tax would due be deducted automatically.
The benefit of £484 as a presumably one off sum on your self assessment was probably a guess by HMRC as to the annual cost of your insurance. If you've paid tax on this, and also been taxed at source on the original premium, you can get the tax back from HMRC. As your employment has now ended, the company's HR department cant make refunds.
Your colleagues benefit of £61 per month suggests he's younger, not insuring a spouse and/or children or all 3.
