True, but the importance lies also in at what stage of ripeness they're picked and the pressing method - cold pressed vs hot pressed, and filtering methods. Blending is pretty much blending and takes place after the pressing process.You started a thread Bitter Tasting Oil Based Dressings and I found this here regarding why and how.
It's a matter of temperature, water content, and how the olives were grown. The method of blending is critically important. Slower and non-machine is better.
Unless the fruit is picked very late or the oil is highly refined bitterness is a characteristic of (almost all) cold pressed olive oil.