Texture alpha channel caused jags

Like the title said,when i have 4 channel will change the compress format from DXT1 to DXT5 automatically.But it was said DXT5 has identical visual results to DXT1 which seems not to be the truth.
Look at the picture below,the left side is Alpha channel,the right side is Red chanel.the alpha Channel is good to me.But in the Red channel,Jags occur exactly on where the alpha channel is.
Anyone know how this happened and how to deal with it?Thanks in advance.

Alpha channel is uncompressed, while RGB is compressed

Do know what exactly you mean but as you can see,the alpha channel do affect the result RGB compressed into.And if i dont have this alpha,the RGB channel compression quality is quite good.

bump!Please help