As a programmer, I feel that word count is a poor metric. You can measure a software engineer's lines of code written per day, but that doesn't tell you anything about their productivity. You can always make a program longer, but actual engineering is implementing a feature in a clear and concise manner.
Being focused on word count alone has perverse incentives. Normal writing tasks like editing and revision should remove words, making a story more focused and easier to understand. You can spend and entire day writing and end up with fewer total words than you started with, but if the story itself is higher quality, I would call that a win.
My advice is not track word count at all. I'm not saying that productive days are bad, but a day spent editing and fixing typos is better than a day writing filler.