The reason for the difference is due to the difference in data at the REST endpoint and is not related to the table in ArcGIS Online.
ArcGIS Pro sample
ID: One
Name: Uno
Domains:
Templates:
Name: Uno
Description:
Drawing Tool: esriFeatureEditToolPoint
Prototype:
Attributes:
TestField: One
ID: Two
Name: Dos
Domains:
Templates:
Name: Dos
Description:
Drawing Tool: esriFeatureEditToolPoint
Prototype:
Attributes:
TestField: Two
ArcMap Sample
ID: One
Name: One
Domains:
Templates:
Name: One
Description:
Drawing Tool: esriFeatureEditToolPoint
Prototype:
Attributes:
TestField: One
ID: Two
Name: Two
Domains:
Templates:
Name: Two
Description:
Drawing Tool: esriFeatureEditToolPoint
Prototype:
Attributes:
TestField: Two