Help me ppl

Hi everyone,

I am final student and  was doing standard costing.I have noticed that formulas given in different books are different.for example the formula of fixed overhead volume variance and capacity variance are different in ravi M kishore than that given in kalra's sir's book or in our institute's book.I know I should not be referring too much books but I am stuck.can any one tell me why the formulas are different everywhere?????

Without prejudice to Pcc level students I request final students to reply specially those who are doing self study as the level of standard costing is different in final..