sphinx coreseek SetSortMode(SPH_SORT_ATTR_ASC, '') 对float 排序设置bug

when I use SetSortMode(SPH_SORT_ATTR_ASC, 'floatVar'), it works unexpectedly. for example, I have a float Var ranging from [-10.0, 10.0] when I SetSortMode(SPH_SORT_ATTR_ASC, 'floatVar', the result is

0

0

0

1.5

2.5

8

-1.5

-2.5

-8

...

why? where I go wrong ?

I just use another solution to solve it ,using

         case 'sentiment':

             $sphinxapi->SetSortMode(SPH_SORT_EXPR, 'sentiment');

         break;

         case 'sentiment.desc':

             $sphinxapi->SetSortMode(SPH_SORT_EXPR, '-sentiment');

         break;

but I still confused why the SPH_SORT_ATTR_ASC didn't work ..