(drop)CommonPrefix: ccs call implementation

Dear list, I am reading up on cost centre stacks. Simon Marlow's "solving an old problem"-slides is the more recent resource I could find. On slide 43 he describes a call function implemented as:

call Sapp Slam = foldr push Spre Slam’ where (Spre, Sapp’, Slam’) = commonPrefix Sapp Slam

However in rts/profiling.c the following is written:

Set CCCS when entering a function.

The algorithm is as follows.

ccs ++> ccsfn = ccs ++ dropCommonPrefix ccs ccsfn

where

dropCommonPrefix A B -- returns the suffix of B after removing any prefix common -- to both A and B.

e.g.

++> <> = ++> <d> = ++> = ++> = ++> =

For the given examples Simon's proposal would result in different stacks: <>, <d>, , and . Has the implementation changed since Simon Marlow gave his talk? If so, is there something I can read on the motivation behind this change? Or do I maybe misinterpret the slides? Thank you, Maarten Faddegon