Approximate is roughly equivilent to cgr and is intened for creating drawings from assemblies is vis mode, reducing the resource requirements to create a drawing of a large assembly. The level of detail in (~acurracy) an appromimate view can be taylored in the properties of the view and in the tools-->options for the generation of the view.
Exact mode is generated from the specification or the brep representaion of the parts. It has a higher memory cost but is exact, it also has a higher level of functionality than approximate (in terms of view types & dimension assosiativity). Approximate mode functionality improved dramatically in R15/R16 in which you can create assosiative dimensions on approximate mode views (but you have to turn it on in the tools options).
In your current situation it sounds like there is somthing in the model that exact mode is having problems with. Where as approximate mode is (sort of) a picture so does have to interpret the model specification, so in this case is producing a better result