139. Under the heading "Adjustment of Retirees' Pensions" the instruction said

"In some countries it is legally required to automatically index pension payments made to retired employees or to periodically review pension payments and make appropriate adjustments. In all other countries where such legal requirement does not exist, benefits of retired employees are to be reviewed periodically for potential increases taking into consideration the inflationary conditions existing under the period under review. In reviewing potential increases to pension for retirees, consideration should be given to the cost effect of any legal requirements to provide comparable increases to vested rights pensions.

"Adjustments to retirees' pensions are to be made in a manner which will avoid establishing precedents that could foster precise expectations on the part of retirees, or in certain cases become legally required.
"...In determining the amount of a pension adjustment, the components of the country's consumer price index should be understood, the wage movement of active IBM employees during the period under review should be considered and the practice of outside companies should be assessed...".
 

The distinction between countries with and without regulatory increases cannot be made with respect to the C-Plan, which has some regulated and some unregulated increases.  The bit about establishing precedents is undoubtedly the explanation of why, unlike (almost?) all other UK pensions schemes, C-Plan pensions are not reviewed at a fixed month of the year.