I think this is where the whole grading system can be disadvantageous to you as an employee.
If you take the "outstanding" categorisation away, how are they saying you did?
Did you just do the job that they were paying you for? Have you gone above and beyond your role, or simply performed your role very well?
As an employer, I pay people to do the job they were employed for. Unless they've done something truly exceptional, why should I pay them more than an inflationary increase? The only way people get a bigger boost on their earnings is to change and develop their role with training and experience and promotion.
The use of the word "outstanding" does feel like you should be expecting more from an end of year pay review.
The other thing that is being factored in to a lot of businesses this year is the increase in NI.
Let's say that you're on £23,000 currently. This year that means they're paying you the £23,000 and employer's NI is £1,918.
They give you a £900 pay increase, so your salary is now £23,900, but from April that employer's NI is £2,835.
A £900 salary increase and the NI increase takes your cost to them from £24,918 to £26,735 which is a 7.29% increase in their total cost.
To give you a 10% salary increase would take your total cost of employment to £25,300 in salary and £3,045 in NI = £28,345 or a 13.75% increase.