Frankly both are true.

For many carriers, fares are uploaded three times a day on weekdays and once a day on weekends. Additionally, analysts are constantly tweaking availability. So the timing of fare transmissions and analyst changes affect the fares you see.

Additionally, some fares are only valid on Mon-Thurs, TuWeSa, any day but Sunday, etc, etc. So the day you travel changes the fares you get.

Long story short, if you see a fare you're willing to purchase, you should almost always purchase it.