Why is index scan run on outer join when not using "SELECT TOP"

If I run the following query without "TOP" in the Select clause then a index scan is run against table tBookingGuest. When I run the query with "TOP(10)" the optimizer chooses to do an index seek against tBookingGuest with the proper covering index. Why would the optimizer behave this way?

SELECT 
       br.bookingid,
       br.roomnumber,
       bg.adults,
       bg.children,
       bg.infants
FROM tBookingRatePlan br
inner join booking b on b.bookingid=br.bookingid 
                      and b.propertyid = @lpropertyid
left join tBookingGuest bg on bg.bookingid = br.bookingid 
                             and bg.roomnumber = br.roomnumber
group by 
  br.bookingid,
  br.roomnumber,
  bg.adults,
  bg.children,
  bg.infants

---------------------- Here is some schema information

**tBookingRatePlan**
  id int identity PK
  bookingid int FK to Booking table
  roomnumber int

**Booking**
BookingId int PK
PropertyId int 

**tBookingGuest**
Id int PK
BookingId int
RoomNumber int
Adults int
Children int
Infants int
*This table has non clustered index 
    (BookingId,RoomNumber includes:adults,children,infants)