Why do we expand tuples in execMain.c?

[Andres Freund]

Hi,

I noticed
                                if (HeapTupleHeaderGetNatts(tuple.t_data) <
                                        RelationGetDescr(erm->relation)->natts)
                                {
                                        copyTuple = heap_expand_tuple(&tuple,
                                                                                
                  RelationGetDescr(erm->relation));
                                }
                                else
                                {
                                        /* successful, copy tuple */
                                        copyTuple = heap_copytuple(&tuple);
                                }

in EvalPlanQualFetchRowMarks, and I'm somewhat confused why it's there?
If it's required here, why isn't it required in dozens of other places?

Greetings,

Andres Freund