Seretide vs symbicort ??

Went to the chest clinic today to see 'prof jones' and ive had my meds changed again. Bit unsure why, i know my asthma is not under controll but I have just been chaged (again) from symibicort 200/12 to seretide 250 to seretide 500 back to symbicort 400/12 i dont quite understand the difference and why i've gone back to symbicort.

Katie xxx