Hi Rob,
That's the quality of the resin casting - for this model not very good.
The better companies like 'Aviattic', 'Lukgraph', 'HPH Models', 'Copper State Models' etc have good resin kits.
However, the smaller and sometimes 'garage' manufacturers use lower grade resin and moulding.
Consequently you can find imperfections, air 'blow' holes, mis-moulds and sometimes like this kit, very thin resin parts.
All part of the challenge.

Here's how thin the tail plane is on this model.

Mike