Why are timing arcs including in cell equivalence?

[maliberty]

Why does equivCells require equivCellTimingArcSets? It is possible for two cells to have the same function but slightly different implementations yielding a different number of arcs. The cells are still logically equivalent even if their electrical behavior is somewhat different.

In rough ascii art the schematics for O2A1O1Ixp5_ASAP7_75t_R vs O2A1O1Ixp33_ASAP7_75t_R in asap7 which are thought not to be equivalent by OpenSTA:

p5                   p33

A1    B              A1    B              
A2    |              A2    |              
+--+--+              +--+--+              
   |                    |                 
   C                    C                 
   |                    |                 
   +------------Y       +------------Y    
   |                    |                 
+--+--------         +--+-+------         
|          |         |    |     |         
B          |         A1   A2    |         
|          |         |    |     |         
+----+     |         +--+-+     |         
|    |     |            |       |         
A1   A2    C            B       C