On numerous occasions I have noticed my vSphere client always has the correct inventory of objects in vCenter whereas the vSphere web client tends to lag behind. While reading Mastering VMware vSphere 5.5 (a great book if you want to really understand how all this works!) I learnt that that’s because the web client depends on the vCenter Inventory Service as a cache between the web client and vCenter whereas the regular client talks to it directly.
The vCenter Inventory Service does two things – one, it caches inventory objects from vCenter so that each time the web client needs something it doesn’t have to ask vCenter (thus reducing load on vCenter); two, it allows for tags. Having the Inventory Service allows for more web client sessions with lesser load on vCenter server.
This also means its a good idea to place the Inventory Service with the web client, not with the vCenter server.