As already said by Barry, that is the effect of roundoff error. You can improve those results by increasing the internal precision used by LTspice's engine with the following SPICE directive:
.OPTION numdgt = 12
this will force the engine to use double precision floating point numbers for calculations (by default it uses single precision).
To add such a directive on the schematic use the menu
edit/spice directive and enter the text above. Then place the floating box that pops up anywhere on the schematic.
Note that this could slow down some complex simulations (not necessarily, it also depends on your CPU, OS, etc.).
For beginners it is reassuring to see simple results not (visibly) affected by roundoff, but keep in mind that as long as computations are done in binary in the CPU there will always be numbers that are "round" when expressed in decimal, but cannot be expressed exactly in binary, and so they will lead to roundoff even in simple calculations, no matter what level of precision the CPU uses internally.