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The distinction behind most of the confusion here

Connection speeds are quoted in bits and file sizes in bytes, and there are eight bits to a byte — the entire source of the gap between an advertised connection speed and the download figure a file manager actually shows. The [msm_tool slug="speed-data-converter"] converts megabits per second into bytes per second by multiplying by a million and then dividing by eight, which is the whole calculation behind why a 100 Mbps connection delivers roughly 12.5 MB per second at its theoretical best rather than the 100 the headline number seems to promise.

Why a gigabyte is not exactly a thousand megabytes here

File size is converted to bytes by multiplying by 1024 three times rather than 1000, matching how operating systems conventionally display file sizes even though storage manufacturers typically use the decimal 1000 convention instead. The difference between the two conventions comes to roughly seven per cent at gigabyte scale, which is enough to notice on a genuinely large file.

Advertised speed is a ceiling, not a guarantee

Network contention, Wi-Fi conditions and distance from the exchange all reduce real throughput below the advertised figure, typically by the largest margin during peak usage hours when the most people are competing for the same shared capacity.

Protocol overhead takes its own share

Headers, acknowledgements and retransmissions all consume part of any connection's real capacity, typically a few per cent even under otherwise clean conditions — a cost that exists regardless of how fast the underlying connection actually is.

The source matters as much as the connection

A slow, throttled or simply busy server caps a download at its own limit no matter how much faster the receiving connection could otherwise go, which is why two downloads over an identical connection can finish at noticeably different speeds depending entirely on where each file is coming from.

This converts figures, it does not measure anything

The result is calculated entirely from numbers supplied by hand rather than from an actual transfer — measuring real throughput requires genuinely moving data across the connection, which is a different kind of tool from a converter working purely from figures typed in.