Torches preventing grass spreading to dirt from adjacent grass blocks

Putting a torch above grass block helps indeed grass spreading. But putting torches on dirt prevents them turn into grass.

I tested it many times, in many occasions. Near 3 seconds after breaking a torch above a dirt block makes spreading almost instant.

 Steps to Reproduce:

1. Put a grass block;
2. Put a diagonal downwards dirt block;
3. Put a torch on the dirt block

Observed Results:
No grass spreading

Expected Results:
Grass spreading.