wireless and wired networks work on slightly different principles...
wired bandwidth is fixed per line and there is only one device per line.
on the other hand a wireless network can be utilized by multiple devices but the network device's bandwidth is fixed. So bandwidth available to user depends on total number of users (or which ever way the network operator may wish to distribute it, the operator may prioritize a certain group of users over others for example).
you may have noticed this thing in large office or university wifi networks that on busy days, the internet is slow and during holiday season/weekend, the internet is fast...