You need to change the list structure of your data. Each sheet gets a data input in the form of a list. Providing a single list for a series of sheets is causing problems.
It all has to do with how the node is expecting the input to be formatted. SheetName and Origin both expect single objects. Data expects a list (usually with sublists). A list of 2 SheetNames represents 2 separate inputs. A list of 2 objects in Data is the expected format for a single input. Thatβs why chopping the list into two separate lists gives the expected result.
Just to expand on that answer, the reason it was writing 1, 0 instead of 10 was because like @Nick_Boyts explained it was expecting two lists (matching two sheets). Trick is that strings are actually enumerable ie. similar to lists, so you can iterate over them. This little nuance of programming languages meant that your single list containing β10β and β20β was interpreted as two lists β1β and β0β and second with β2β and β0β.