i think what we may be missing here is @gaugustfrank is actually talking about the nashville numbers system, and why in a set key (major or minor), certain chords are major and some are minor, and this is what he/she is confused by. why are some of them major and some minor. or maybe not the numbering as such although that may help, but the pattern of majors and minors in the key, which in this system are denoted by either capital or lower case roman numerals.
rather me than typing out a load of confusing bumph, it might be helpful to go googling the nashville numbers system and see if thats what youre aiming to understand?