Why did Cheggs expert convert byte to kilo byte by dividing on 1024 not 1000 ?

Resolution given 1920x1080 pixels Total number of pixels (1920 x 1080) pixels- 2073600 pixels There are 3 bytes per pixel So 2073600 pixels will have (2073600 x 3) bytes 6220800 bytes Frames rate per second 60 FPS Download rate (6220800 x 60) 373248000 bytes/sec Since video is 1 minute (60 second) long Therefore, total number of bytes in 1 minute video (373248000 x 60) bytes 22394880000 bytes Now, converting bytes into kb or kilobytes Since, 1 kb 1024 bytes Therefore, 22394880000 bytes (22394880000/ 1024) kb-21870000 kb a) Time taken to download this video over 28 kbps modem- (21870000/28) sec 781071.43 sec (781071.43 60) minute 13017.86 minute (13017.86 60) hours 216.96 hours b) Converting kb into Mb Since 1Mb 1024 kb Therefore, (21870000/ 1024) Mb-21357.42 Mb Time taken to download this video over 15 Mbps cable modem- (21357.42 /15) sec 1423.82sec- (1423.82/60) min 23.73 min c) Time taken to download this video over 100 Mbps Ethernet-(21357.42/100) sec 213.57 sec- (213.57/60) min-3.56 min d) Converting into 21357.42 Mb into equivalent Gb Since 1Gb-1024Mb Therefore, 2135.74 Mb (21357.42/1024) Gb 20.86 Gb Time taken to download this video over 1 Gbps Ethernet- (20.86/1) sec 20.86 sec