Spouse visa from 5year to 10 year route conversion

Hi.
My name is muhammad i am pakistani national , my wife is british citizen and we have a 2 year old baby which is also a british citizen. I came to uk in dec.2014 on spouse visa on 5 year route. In aug.2017 i applied for my visa extension.
The home office granted me visa of 30 months on 27 sep. 2017 but they put me on 10 year route saying that my income is less than 18600.
But i had have calculated my income before applying visa. Which was 9400/ 26 weeks which was equal to 18800/ year. I dont know how home office calculated it as 9037/6 month which is equal to 18074/year. Moreover they dont have given me any right of appeal.
Please tell me how can i prove that my income is more than the required (£18600). And how can i come to back on 5 year route from 10 year route.
Thanks regards,
Muhammad

I think you are doing a non salried job with flexible salary every week or month.Home office take the lowest amount to get the average of salary. In my opinion that's why they put you on 10 year route.

To calculate this annualised average for non-salaried employment in Category A the following calculation should be used:
(Total gross income from employment held throughout the 6 month period, divided by 6) multiplied by 12 = Income from non-salaried employment that can be counted towards the financial requirement."