Seems quite simple to me, dick. Your mates started work in July and overpaid their tax for some months. Instead of getting a lump sum tax rebate they paid no tax for 2 or 3 months.
You started in January which gives the tax man a lot longer to get your tax sorted out in the same tax year. If you don't get a lump sum rebate you will pay less tax until you are square.
Your mates are not getting something you aren't - you will all have paid the correct tax over the tax year.