Actually, prices don't fluctuate much at all. What's going on here is that airlines sell tickets in several fare classes, with different privileges and restrictions in each. The initial allocation of tickets to fare classes is based on the airline's best estimate of how many tickets of each type they'll be able to sell. For example, someone might be willing to pay the highest economy fare (typically called Y) if necessary, but will buy a lower-priced ticket with few restrictions (such as the modestly discounted B fare) if it's available. It's to the airline's advantage not to have any B tickets available when this person shows up. They can't manage it that precisely, of course, but they do limit the number of tickets they sell at lower price points in order to force people to pay more or to take less popular flights for which cheap tickets are still available.

The allocation of seats to fare classes is not fixed when a flight is loaded into the reservation system. It's fine-tuned as time goes on, based on how well that flight and other flights are selling. Therefore, the lowest fare for a given flight may seem to fluctuate when it's almost always a low fare class being sold out, becoming available again, and then selling out again. If you check the specific fare class associated with the lowest fare each time, you'll be able to confirm this.

In your case, that $1100 fare is almost certainly in one fare class while the $1400 fare is in another. Fare details will be able to tell you what you get for the extra $300, besides a bigger credit card balance: perhaps it's the ability to change your flight for a fee vs. no changes at all, perhaps it's more frequent flyer mileage credit, perhaps something else.