This most known and used vegetation index is a simple, but effective VI for quantifying green vegetation. It normalizes green leaf scattering in the Near Infra-red wavelength and chlorophyll absorption in the red wavelength.
Values description: The value range of an NDVI is -1 to 1. Negative values of NDVI (values approaching -1) correspond to water. Values close to zero (-0.1 to 0.1) generally correspond to barren areas of rock, sand, or snow. Low, positive values represent shrub and grassland (approximately 0.2 to 0.4), while high values indicate temperate and tropical rainforests (values approaching 1).
There is nothing wrong with your values, and you can compare them as they are now because the NDVI is already normalized (that´s the
So to answer your question directly:
Do I need to make all the min-max values the same to be able to compare all the NDVI's and, if so, how do I do this?
Depending on what you are trying to achieve, but if you simply want to compare the NDVI of an AOI during different periods then you don't need to make the min-max values the same.
If you are trying to normalize the colors for the visualization, just make sure that you set the same break rules for the stretch in every raster.