Why is my MacBook Pro showing the low battery screen while at 100%?

Your battery is failing; it's not holding a charge It's time to have it replaced.

The symptoms you've described is indicative of one or more failed cells. Your battery is going to "report" back to the SMC that it has:

• 7496 milliampe hours at full charge
• 3113 milliampe hours remaining (calculated value)
• 451 charge cycles
• it's putting out approximately 12.3 volts (measured value)

As much as it can measure at a single point in time, it cannot "look into" the battery to see if there's capacity to hold a charge - in other words, there's no "dipstick" to measure capacity within the cells themselves.

When there's a bad cell, your charge will go from "good" to "bad" with no warning (i.e. 20% to nothing) because it cannot account for the missing capacity.

Also, given the age of the MacBook (3+ years) it's well within it's lifespan of 3 to 5 years.