I am going to take a guess at this. This is the opposite of what I expected. As I recall, the parameters for +/-I, etc. are derived empirically. So, if a reaction were performed, a nitration of trimethylanilinium and anilinium chloride for example, the trimethylanilinium chloride would have a greater amount of meta substitution and the reaction rate slower.
I suspect that while the electrophilic substitution reaction (or other reactions in which the -I effect is being measured) is essentially a kinetic process, anilinium ion can also participate in a deprotonation reaction as well. Although this is a thermodynamic process in which the anilinium ion is favored, aniline itself will have a +I value and can incrementally increase the apparent -I value.
I am not inviting anyone to believe this. I am only making a guess. If more data were presented, my guess could be more easily verified, refuted, or another explanation suggested.