Wayne is correct...even if all colors are in gamut, RelCol and Percept will not match.
True. How Perceptual rendering functions is up to the profiling software. In general, the goals are to make a visually pleasing conversion between color spaces. This implies minimizing banding as colors clump near the gamut boundary and, for some implementations, smoothing tonal transitions, opening shadows, etc. All this color manipulation means
all colors are altered to some extent, otherwise you will get banding in other areas.
Rel. Color prints in-gamut colors unchanged. What happens at gamut boundaries again depends on the profiling code. Some implementations collapse similar colors into one, while others take more liberty with the rendering intent and smooth transitions even if it is not mathematically the most accurate approach.
Something older versions of Photoshop were particularly poor at was displaying the effect of contrast range compression. The IQ 180 back I used today has a dynamic range of ~13.5 giving a contrast ratio of more than 12000:1. Good ink jets printing on glossy stock reach 300-400:1 while matte paper often has a print contrast under 100:1. Traditional wet lab printing as still used by Fuji Frontier and similar printers max out around 100:1 as well. Obviously something has to give.
Photoshop's simulation options attempt to show the effects of contrast range The gamut warning often does not reflect what is actually happening.
A further issue is what source color space is assumed perceptual algorithm. As far as I can tell, i1Profiler uses either sRGB or a slightly expanded gamut version thereof. Current ICC specs attempt to address this problem, but that has been the case for years. Don't hold your breath.
Images with significant amounts of out of gamut colors, particularly ones in larger color spaces, benefit from device link profiles. Unlike normal profile conversions a device link can be created using the actual image gamut for the source color space rather than the much larger mathematical abstraction of ProPhoto, etc. If you are comfortable going the command line route, Argyll is a powerful option.