Internet speed is sold in bits per second, file size is shown in bytes - an exact 8x difference. A "100 Mbps" plan really moves data at 12.5 MB/s, not 100 MB/s.
ISPs advertise speed in megabits per second (Mbps) because it's a bigger, more impressive-looking number. File sizes and download progress bars show megabytes (MB) because that's how storage is measured. There are 8 bits in a byte, so "100 Mbps" is really 12.5 megabytes per second of actual throughput - a 4.7GB file at that speed takes about 6 minutes 16 seconds, not the 47 seconds you'd get from naively dividing 4700 by 100. This one mix-up explains more "why is my download slower than my internet speed" confusion than almost anything else.
Even with the bits-to-bytes math right, real transfers rarely hit the full converted figure. Some of the connection carries protocol overhead rather than your file - packet headers, acknowledgements, encryption - which typically costs a few percent. Wi-Fi adds its own losses, other devices share the line, and the server on the far end has to keep up too. A practical rule: treat the calculated time as the best case, and expect real-world transfers to land somewhere modestly above it. If a transfer is running at half the calculated speed or worse, that's usually worth investigating - the common culprits are Wi-Fi distance, a busy server, or a plan speed lower than remembered.