PVC uses what's called "Interference Fit", this design has very high tolerances which makes it near impossible to dry fit PVC.
Why you ask?
Well the "Interference Fit" combined with the solvent is what allows PVC to create a sold bond/connection. When you apply the solvent and push the two pieces together the high tollerance is creating friction between the two pieces which causes the PVC to actually melt. So through this process you are experiencing "Thermal Expansion". This is what explains why you have to hold the joint tightly together as it wants to push out.
Push the joints together >>> creates friction>>> PVC melts and expand>>> the melted PVC joins together>>> it then cools and contracts>>> a new bond/weld is created.
Hence why you use PVC solvent and not PVC glue.