When creating an enclosure in Assembly Simplification, if the default templates have different units than the assembly itself, then the units will not be the same. In other words, if the assembly was created using ANSI inch and the default templates are metric, when you add an enclosure the units will be mm.
Solution
In Solid Edge Options, under Helpers, select "Set Default Templates". Change the default templates so the units are the same as the active assembly.
Notes and References
Hardware/Software Configuration
Platform: INTL64
OS: windows
OS Version: 10_1909
Product: SOLID_EDGE
Application: ASSEMBLY
Version: V221MP2
Function: SIMPLIFY
Ref: 001-9971179