Barry's guess that your numbers are actually text, not numbers, is probably correct. They may look like numbers to you and me and to the + - * / operators but most numeric functions will ignore them as text. It is interesting that categories Maximum accepts textual numbers but Subtotal does not. I tried it out and that is, in fact, the case. Either the cells that contain the numbers are formatted as text or the numbers themselves have some problem such as using a comma for the decimal when your system settings say it should be a point.
A quick test is to turn off categories, select that column of numbers, and look at the bottom of the window at the value given for Sum, Max, Min, etc. If those values look wrong, you probably have a problem. Or select a cell that you think is a problem and see what it says in the bottom left corner of the window. If it says "actual" it is a number (or a date). If it says "text" it is text. You can go down your entire column that way if it isn't too long.
If your column is really long, temporarily create a new column B. Assuming your first data row is row 2, in cell B2 put the formula =COUNT(A2). Fill down to the rest of the rows. Any row that has an actual number (or a date) will get a 1. Any rows that have text will get a zero.
If there is no problem with your numbers, they are just formatted as text, select the cells/column and change the cell format to a numeric format. You might try that as the first step, actually, before doing anything else. Typically, numbers should be numbers when you enter them in the cell unless you have formatted the cell as text or there is a problem with the number itself, making it not a number.