It's a business, so they update the maps, where it makes the most sense from a business point of view. Making quality maps is a very labor intense task and looks simpler from the outside, than it actually is. google for instance has more people working on their maps than TomTom has employees in total (which is betweem 3k and 4k). That's how intense it is...
(full disclosure: I worked in TomToms maps division for 7 years).