Without getting into details about digital modulations, yes, parameters such as signal strength, network congestion, user speeds, etc. do affect transfer rates.

Without getting into details about digital modulations, yes, parameters such as signal strength, network congestion, user speeds, etc. do affect transfer rates. The speeds quoted by phone providers are always the maximum transfer rates allowed by the communication standard used by the network. To give an example with WiFi networks. Even though a router might support 802.11n and have a maximum speed of 144Mbit/s, if you're far from it, your transfer speed could only be 11Mbit/sec.