I think it has to do with how the dimensions of the tables are set. The dimensions of table for the preview window are large enough where the graphics of your sigs canco-exist side by side. However, the actual dimensions of the table for the signature block is considerably smaller. Just eyeballing it I would say the sig graphic is between 2/3 and 3/4 of the signature block area.
Lets say that the signature is 400 pixels wide. If the area in the preview for the of the sig is set for for greater that 800 pixels then it should display side by side. But if the actual signature are is set at 700 pixels then they will try to wrap.
Also, and I may be wrong about this, you can only have four lines in the signature. If that is the case, when you actually try to use the sig, it will break you lines so that the graphics are on one per line and thus you go over the limit of number of lines you can have.
Lets say that the signature is 400 pixels wide. If the area in the preview for the of the sig is set for for greater that 800 pixels then it should display side by side. But if the actual signature are is set at 700 pixels then they will try to wrap.
Also, and I may be wrong about this, you can only have four lines in the signature. If that is the case, when you actually try to use the sig, it will break you lines so that the graphics are on one per line and thus you go over the limit of number of lines you can have.