I have couple of 2600 routers , I did test for all of them

Router (config )# enable password xxx

Router (config )# enable secret xxx

Router(config)# enable secret level xxxx

I logged in into level 15 (password xxx), then I quit from default level (15) to user mode , then I logged in into level 10 ( password xxx ), then when I quit from level 10 , I was not able to access default level (15) again,,,why ?