As one probably knows, the MONI and the newer SWNCMONI database tables are cluster tables and shouldn't be read directly, use new FM SWNC_COLLECTOR_GET_AGGREGATES
for that.
Nevertheless, if you still want this:
TYPES: tt_memory TYPE TABLE OF swncaggmemory.
DATA: ms_monikey TYPE swncmonikey,
dummy TYPE tt_memory.
FIELD-SYMBOLS: <tab> TYPE ANY TABLE.
ASSIGN dummy TO <tab>.
ms_monikey-component = <instance_id>.
ms_monikey-comptype = 'NW Workload'.
ms_monikey-assigndsys = <host>.
ms_monikey-periodtype = 'D'.
ms_monikey-periodstrt = '20200713'.
IMPORT datatable TO <tab>
FROM DATABASE swncmoni(wj) ID ms_monikey
IGNORING STRUCTURE BOUNDARIES.

As you can see that data for PFCG differs from ST03n in spite it is called for the same date.
Answering on your second question: why it differ?
It may depends on data aggregation setting for memory profile

also try to play with aggregation period. Actually I also wasn't able to find correspondence between them.
Many useful info about ST03 is here
https://blogs.sap.com/2007/03/16/how-to-read-st03n-datasets-from-db/