Some words are way more common than others, and they are much more likely to be found in your passphrase if you didn't use an external random index generator (be it a set of dice or /dev/urandom), because of availability bias: you're more likely to chose common words, or even worse, uncommon words pertaining to your interests.

Diceware uses a smaller set (about 8K words), but if you select them truly randomly, you'll get close to 13 bits of entropy per word —guaranteed. I wouldn't cry too much over the loss of 2 bits per word, because Diceware words tend to be short.

Then again, even a word in a grammatically correct sentence is likely to have more than the 6-7 bits of entropy a character may have. I wouldn't bet on anything above 10, though.