The container (bag) frames have their own custom code which uses SetPoint to adjust them relative to each other, but frames like the CharacterInfo and Merchant use common functions from UIParent.lua. To make use of this common code you need to define UI Panel layout attributes for your frame after you create it. You can do it in Scripts/OnLoad section of XML or right after you call CreateFrame in Lua. Finally, you'll need to use ShowUIPanel(frame) and HideUIPanel(frame) instead of frame:Show() / Hide();