Not all Wi-Fi devices will be capable of providing the same speeds you would get with a wired connection.  Refer to your devices manual for specification regarding the Wi-Fi standard being used and its limitations.
It is recommended that only a single device is connected to the internet when testing your speed to help ensure you are getting the most accurate results.  You also want to ensure that any internet activity such as downloading or steaming is paused for the duration of the test.
Learn more about how you can boost your wifi signal here.