The scale 0-255 is not an absolute scale with defined "distances" in a histogram.
So, ProPhotoRGB Adobe98 RGB and sRGB will all show a histogram with values between 0-255 but the RGB values derived from a sRGB histogram will not be the same as that derived from an AdobeRGB or ProPhotoRGB histogram even though the actual colour is the same.
So those values are only relative to the gamut from which they are derived.
To expand on what Tony has said, the color values in Adobe RGB and ProPhotoRGB are not absolute, but are relative to the space in which they are defined. The L*a*b space is device independent. However, when one renders into L*a*b, the values are not what was present in the actual scene but depends on how they were rendered into the space.
When evaluating the histograms for clipping, it is helpful to determine if clipping was present in the raw file or occurred via overzealous development settings. The first step is to look at the histogram of the raw file. Rawdigger is an excellent tool for this purpose. Clipping in the rendered file can be due to luminance or saturation clipping. Generally speaking, saturation clipping in ACR is indicated by one or more of the color channels showing a cliff like cutoff on the right of the histogram.
Here is a scene rendered into ProPhotoRGB with ACR. The red channel is short of clipping.
The raw histogram shows that all channels are exposed to the right without any clipping. The red channel is considerably short of clipping, but when white balance is applied, the red histogram will move to the right.
Rendering into Adobe RGB with the same settings shows saturation clipping of the red channel.
With the latest version of ACR, one can render into L*a*b, which separates luminance and color and is device independent. The histogram is here and shows a full range of luminance without clipping. The color channels are far from clipping.
Overexposure rendering into ProPhotoRGB does show clipping, which is largely luminance clipping. The red channel appears minimally clipped.
The L*a*b histogram shows that the color channels are free of clipping, while the luminance is clipped.