Many different factors that influence signal strength and quality; these factors include, but are not limited, to the following:
Proximity to a cellular tower
Tower load
Physical barriers, such as mountains, buildings, or trains
Competing signals
Weather
Signal going through a cellular repeater
Signal strength and quality numbers do not incorporate all of the relevant factors. Keep in mind that measurements of signal strength and quality for a specific moment do not reflect the stability of a connection, as these values will vary as conditions change.