Hi Mike,
Before investigating that output, please check below.
1.Are they have composite indexes?
If so, how did you update their statistics?
update index statistics or update statistics?
For ASE 15.7 you have to use 'update index statistics' command.
2.Below set options will help you to understand it.
2.1.check missing statistics
set option show_missing_stats on
go
2.2.enable below options too.
set statistics io, plancost on
go
Thanks,
Jongun Seo