syntax-region's odd behavior

syn_region.vim

syntax clear

syntax match expr_name /\K\k*/ nextgroup=B

syntax region outer
	  \ start=/(/ end=/)/
	  \ contains=inner

syntax region inner
	  \ start=/(/ end=/)/
	  \ contains=expr_name
	  \ contained

highlight outer guifg=#00ffff
highlight inner guifg=#ff0000
highlight expr_name guifg=#ff00ff

" Expected:  ((aaa))
"            ^     ^ outer
"             ^   ^  inner
"              ^^^   expr_name
" Actual:    ((aaa))
"            ^^   ^  inner
"              ^^^   expr_name
"                  ^ outer

syn_region_fixed.vim

" Using matchgroup to avoid above behavior.

" :help :syn-contains says:
" > The contained groups will also match in the start and end patterns of a
" > region.  If this is not wanted, the "matchgroup" argument can be used
" > |:syn-matchgroup|.


syntax clear

syntax match expr_name /\K\k*/ nextgroup=B

syntax region outer
	  \ matchgroup=outer_parenthesis
	  \ start=/(/ end=/)/
	  \ contains=inner

syntax region inner
	  \ matchgroup=inner_parenthesis
	  \ start=/(/ end=/)/
	  \ contains=expr_name
	  \ contained

highlight outer_parenthesis guifg=#00ffff
highlight inner_parenthesis guifg=#ff0000
highlight expr_name guifg=#ff00ff