Good question.

It seems that Tosfos does not inherently require such a great distance between the rows of seeds; a distance of three Tefachim would have sufficed. However, according to the way that Tosfos explains the formation of the seeds in the Arugah, there is no other way for Tosfos to explain how multiple Arugos could be sewn together in one garden.

The four sides (as opposed to corners) of the center Arugah cannot be surrounded by Arugos, because there would be only two Tefachim between the facing rows of seed (such as the eastern side of the center Arugah, and the western side of the Arugah to the east of the center Arugah). Rather, the surrounding Arugos must be placed at the corners. However, once this is done, there are more than five Tefachim between the rows, as you point out. That distance results from (a) the necessity to put the other Arugos at the corners of the center Arugah, and from (b) the necessity to arrange the seeds in each individual Arugah in the particular formation that Tosfos prescribes.

Y. SHAW