They are exactly what they say: Total number (in all tables) vs Present number (in this table). For a normal field, there is no difference.
But if the field is a key between tables, it may make a difference. For example, if you have values A and B in one table, and A and C in a second table, the total number of values is 3, but neither of the tables has more than 2 present values.
I had the same question, so thank you for clarifying. I didn't find the terminology to be self-explanatory... The Help web page dedicated to the Data Model Viewer could add a couple sentences about this.