Why are dates sometimes collapsed in filter mode and other times they aren't?

I know the 1st thing you might ask is, what does the date look like in the cell. I've noticed this over many years of working with the filter mode. Its just sometimes they are collapsed and other times they aren't. Sometimes the dates look exactly the same. Usually I just ignore it and go on.

I totally know its due to formatting. I'm just trying to figure out... Under what circumstances do the dates automatically collapse and under what circumstances do they not.

For example, I'm working with a sheet of numbers today, the data originated from a CSV file. There are 2 date columns, Both visually appear with the format of dd-mm-yy, and the characters are typed are mm/d/yyy After turning on the filter 1 column appears collapsed and the other does not. I tried to format both columns as short date. they both then change to mm/dd/yyy. But still, 1 collapses and the other does not. I checked the format for individual cells in each column they both appear to have the same format.

Piddly, irksome problem that I've never figured out.

note: I tired to import the file and format each column as date. I also opened the csv, copied the data and pasted to a new xlxs file. Same thing happened.

Not sure without seeing your data, but keep in mind that real dates are right aligned in their cell, while text looking like dates are left-aligned ( when no manual alignment is applied). Formatting will not change that, it only changes the way Excel shows the value/text to the outer world.

I think I discovered my own answer.
When using the Filter Mode for dates, EXCEL using "grouping" and by default shows the top level of "grouping".

When using the Filter Mode on a sheet containing dates... Sometimes the dates will appear in the filter box as collapsed within its own month, sometimes the filter box will show the dates expanded as a list.
EXCEL uses the filter box to group dates.

• It seems that if the filtered column contains only dates of a singular month, they are expended by default. Showing the dates as a list or, within the "group" of a singular month. Because there is only 1 "group".
  
• If the column contains dates from more that 1 month or any other kind of data, the list appears collapsed by default, grouping the dates into their "group" by month and any other kind of data below the months or "groups". Since there is more than 1 "group", it defaults to the top level of "grouping".

My sheet had 2 columns of dates. 1 column had dates from a singular month. Excel expanded the group be default. The other had dates from 2 different months and additionally 3 cells with "--", therefore that filter box showed the dates collapsed into their own month and the "--" listed below the 2 months.

There are probably other specific behaviors for the filter mode that I haven't discovered yet. But at least I understand this now.