If an employee receives increases in compensation in the last four years of service which are not related to promotion and which exceed the average increase received by other employees of the same employer holding comparable positions, the excess shall be excluded when computing the average final compensation if the Board finds, after consideration of all circumstances, that the primary purpose of the salary increase was to increase the retirement benefit of the employee. If there are no employees of the same employer holding comparable positions, the increases may be excluded from the average final compensation if they exceed the average percentage increase received by all other employees of the same employer. Creditable compensation assumed to have been received for the purpose of purchasing service shall be excluded from a member’s average final compensation.