Secret Key value for decimal versions over 127
The reason is historical.

The 'version' byte was originally really just a version number. Satoshi probably intended to introduce more types of standard scripts to addresses. There were comments in the code about upgrading an address from one version to a new one even.

In that vision, to not restrict the options for future versions, I chose to make the private key just set the highest bit, leaving still 127 possible future versions.

Only one new version was ever introduced (P2SH), and since it is so powerful (it can encode any script indirectly), no other versions ever appeared.

So the short answer is: it does not matter at all.