Sure, the Prius has very high fuel economy ratings, but they are for an exact driving pattern designed to represent “average” driving formulated by the EPA.

It is very easy to not be average.

If you drive ~68mph in typical freeway traffic with limited stop and go driving on surface streets - about 50/50, then yeah, you can get pretty close to the EPA ratings.

If you practice very conservative driving and hypermiling techniques, you can do better than EPA.

If you have a lead foot, or if you keep up with the traffic doing 75-80mph, drive a bunch of short trips, deliveries, errands, rideshare, or you use your A/C a lot in the extreme heat (and you should here in Phoenix), your mileage may be significantly below EPA.

Here’s an example: My son drives a 2008. It has a new battery with only 30,000 miles on it, a brand new 12V and all maintenance current. He’s not a lead foot, but he does tend to accelerate to speed fairly quickly, drives a whopping 4-5 miles round trip to work and 10-11 miles round trip to school. He averages 31-33mpg year round. When I drive it across town, it readily pops up into the 42-43mpg range.

Yours truly, who might be a lead foot and might be seen keeping up with traffic trying not to get run over or flipped off by the 4x4 drivers, rarely gets above 37-38mpg.

Driving style and conditions dictate mileage more than any specific issue with the car.

Good mileage:

<70mph on the freeway
Mixed driving with a 40-60% split between city and highway.
Moderate acceleration and extended braking.
Conservative driving.
Use A/C in AUTO mode and adjust thermostat as needed keeping ALL vents in the car open in an attempt to cool the entire interior (or else the battery sucks in warm air for cooling).

Bad mileage:

> 70mph on the freeway
Short trips or “errands” where you drive a few miles stop some where for 20 minutes, return to car and rinse/repeat a few times - this will cook the battery and mostly disable the hybrid system killing performance.
Rideshare
Deliveries
Jack rabbit starts (as much as a Prius can) and rapid acceleration.
Hard/late braking.
HEAT (A/C use increased, but don’t you dare turn it off. If all 5 people in the car, even if they’re only imaginary, aren’t cool and comfy, your battery is too hot impacting both economy and battery life.
COLD. The only source of eat is the engine. If you need heat, it will run the engine more.

Hopefully, I’ve painted a picture that gets the point across that your mpg depends on WAY more than how well your car or hybrid battery is doing. Sure, a poor state of tune, neglected maintenance and a weak hybrid battery can adversely impact fuel economy, but the above factors have a much greater influence over the life of the vehicle.

More than you ever wanted to know about EPA testing.

Lastly, ratings were changed in 2008 and 2017 in efforts to better reflect real-world conditions. In both cases, those numbers resulted in LOWER ratings than those that were originally on the window sticker when sold. Ratings are at http://www.fueleconomy.gov, and it also lists the originals, e.g., the 2007 Prius pulls up as 48/45 city/highway to current standards, but when it sold new, the sticker was 60/51 city/highway.