Ro membranes are rated at only around 95 to 97% efficient. So if you have pretty hard water, you get some measurable hardness out of the membrane. The follow up DI filter removes the residual hardness.
My RO unit doesnt not have a DI filter and I have about 600 TDS going into it and about 30 TDS coming out.
As to whether that small amount causes problems for shrimp, I cant tell you. BUt since I remineralize my shrimp water, I'd say no, it doesnt cause problems.