Query returns different result in postgresql

I am working on one project in which I am using postgresql.

I have following values

A     B -------------- 18815   0 29187   0 29494   0 28995   0 0       0 25762   3517 29291   0 29612   0 29331   0 30004   0 

Following query I am using to get my result.

select to_char(interval '1 second'*(A+B),'HH24:MI') as resulthours  For Example - select to_char(interval '1 second'*(18815+0),'HH24:MI') as resulthours (it gives result = 05:13) 

same way I got following result for all the values from both columns A and B

Result  05:13 08:06 08:11 08:03 08:07 08:08 08:13 08:08 08:20 

total – 70:29

now the issue is, expected result should be 70.55 , but I am getting gives 70:29