Yes it is strange behavior. If you inherit group one by one, then it will be in list (like roles). But if you have standalone at least one, then it will be like options. It cause impossible define exclusive roles.

If you need middle level, for example "Project/Specialist", then you need inherit Specialist from User and Manager inherit from Specialist.

<record id="group_xxx_user" model="res.groups">
    <field name="name">User</field>
    <field name="category_id" ref="module_category_yyy"/>
</record>

<record id="group_xxx_specialist" model="res.groups">
    <field name="name">Specialist</field>
    <field name="category_id" ref="module_category_yyy"/>
    <field name="implied_ids" eval="[(4, ref('group_xxx_user'))]"/>
</record>

<record id="group_xxx_manager" model="res.groups">
    <field name="name">Manager</field>
    <field name="category_id" ref="module_category_yyy"/>
    <field name="implied_ids" eval="[(4, ref('group_xxx_specialist'))]"/>
    <field name="users" eval="[(4, ref('base.user_root'))]"/>
</record>

, where xxx - name of module, yyy - name of category.

Or define it through web interface: "Settings/Users/Groups/Inherited"