Q400 single engine climb data and service ceiling
 

What are the difference between the single engine service ceiling (table 4.2.2 AOM), single engine cilmb data (5.8.2) and single engine cruise data (5.8.4) :
the three gives you different altitudes in the same conditions:
for example : in ISA conditions at 28t you can according to the single engine climb data climb to FL160 , and according to the single engine cruise data you can cruise at FL200 and according to the single engine service ceiling you can drift down to 17480ft . why is that so different ?