There are two elements to the "deal" - the salary sacrifice element and the BIK element.
A BIK is something that your employer pays on your behalf and is looked at as "additional pay". This is for things like cars but also private medical insurance. If, for example, your employer paid £100 a month in private healthcare, you'd be taxed on the basis of "earning" an extra £100 a month.
A salary sacrifice allows you to pay for something using your pre-tax salary. In your case £1,250 a month comes out of your gross pay, meaning you're not paying income tax or NI on this. If you were to lease a car yourself without the scheme, you'd be leasing it out of net pay, after tax, not before.
So, while you benefit from not paying income tax and NI on the amount of salary you're sacrificing, your employer still needs to report your company car as a benefit in kind.
This is calculated based on the list price of the car and the appropriate BIK %. If your XC90 is a fully electric car, this is currently 2%, but will be increasing by 1% per year over the next couple of years.
If, for example, therefore, your car has a list price of £60,000, you'll get an annual BIK of £60,000 x 2% = £1,200. You'd therefore be paying tax (let's assume you're a 40% taxpayer) of £480 per year = £40 per month.
This is a good summary - https://www.carwow.co.uk/guides/buying/electric-car-salary-sacrifice#gref
The example they use is someone leasing a Tesla for £550 a month.
If you are a higher rate tax payer, that £550 of earnings would normally be taxed at £220, leaving you with £330. By sacrificing £550 of salary you're effectively paying £330 for a £550 car. (NI is an extra saving)
You would pay tax on the BIK of £45,990 (list price) x 2% = £920 per year or £76.65 per month x 40% = £30.66.
In summary:
If you are sacrificing £1,250 a month from your pay, but are paying £850 "tax" on the BIK per month, I think it looks way off.
Using my example of a £60k list price above, you should be down your £1,250 gross salary per month and £40 a month in tax (based on a £100 BIK per month).
I'd recommend speaking with your payroll department or HMRC.
What's your tax code on your payslip?