Most modern scanners produce level scans now. This is typically an attribute of the raw scan data. It has been my experience that Recap does not honor this attribute and has no problem tilting level scan data in the XZ or YZ axis if it belives there is a better fit that way. Most other registration software does not do this or at the very least will warn you when it has done it.
On smaller proejcts this is not usually an issue but can be catostrophic on larger projects. For exampel a 1/4 degree tilt becomes a huge problem when extended out over a 1000' or more.