Tiling pattern in Illustrator, stroke and fill difference

Hello, Would you tell me please why this is occurring to my tiled, expanded and merged pattern in Illustrator? See how the line is thinner at the top? The whole pattern is the same stroke/fill.  Up close it looks fine, but zoom back and it looks like this.  Is it an Ai view glitch or do I need to add a step somewhere?  Thanks in advance. J