MA BPP Revision Kit 2020-2021, Budgeting, 10a.3, Pg79

Greetings sir,
Hope you are stronger by time,

Something of contention I came across:

10.a3,
I calculated the Labour for 83% just the way I did for material (2120/80 x 83%), and then I calculated the Production overhead using the high low method. The final answer I got as a result of this procedure was: 8755.5 ~ 8756.

83%
M = 2490
L = 2199.5
O/H = 4066 (v.c = 2, f.c = 3900)
Total: 8755.5

However, they preferred somehow to calculate both the labour and overhead together combined using the high low method in order to find the variable cost – strange, and reached a different answer.

Please assist with your great knowledge.