Reformated the query to make the two lines evident.
This bug appears when the coordinates of the center upper left pixel are very precises i.e. -85.750000033333492
A first problem is that the order is different.
A second problem is that the numbers are also different when the precision is high. I guess this problem is related to the fact that bbox are stored as float4s in PostGIS and our version of ST_Envelope returns the float8s version. I think we should return the float4s bbox. I have to test further though.