What do you exactly mean when you say that a COMPUTE and an ABSOLUTE are the same thing? From the rrdcreate man page I'd say they aren't:
ABSOLUTE is for counters which get reset upon reading. This is used for fast counters which tend to overflow. So instead of reading them normally you reset them after every read to make sure you have a maximum time available before the next overflow. Another usage is for things you count like number of messages since the last update. COMPUTE is for storing the result of a formula applied to other data sources in the RRD. This data source is not supplied a value on update, but rather its Primary Data Points (PDPs) are computed from the PDPs of the data sources according to the rpn-expression that defines the formula. Consolidation functions are then applied normally to the PDPs of the COMPUTE data source (that is the rpn-expression is only applied to generate PDPs). In database software, such data sets are referred to as "virtual" or "computed" columns.
I am new to both rrdtool and RRDTool::OO, so if I am missing something obvious please bear with me and tell me ;)
In theory, there is no difference between theory and practice. In practice, there is.