Hobbes spelling inconsistency

I was reading Chapter 2 of Leviathan, on Page 17 of my edition (but I'm not sure if most editions have the same numbering). Hobbes spells the same word two different ways, "brain" and "brayn".

This is the first instance:

... in sense, the Brain, and Nerves, which are the necessary Organs of sense ...

And this is the second:

... for the connexion they have with the Brayn, and other Organs ...

It'd be strange for either one to just be a spelling mistake, since a "y" is a pretty obvious difference, and the quotes are pretty close together. Is my edition just weird, or does this actually mean something grammatically?

By the point this was originally published in 1651, English had already come so far from its Middle English days: most of the words are spelled the same, even between different books and different authors! According to the OED blog:

By the mid-seventeenth century printers followed general principles of spelling much like the present ones. [...] The spelling of nearly all individual words was also identical with present-day forms in printed books.

But it was not uncommon to see authors spelling some words inconsistently even at that point. In fact there are some words still like this today (grey vs gray).

I even found some examples that are later than yours. From Citt and Bumpkin (1680):

• Cock-brayn'd Zelots
• Cockbrain'd Linnen-Draper

(I noticed it also uses both Publick and Publique.)

The muses melody (1680):

• Then that within his Brayn
• And ev'ry mans brains are in Alto.

(This one has both Ladies and Ladyes.)

As for why the Y spelling came to be, again from the OED blog:

The letter y was commonly used for the vowel i, especially in the vicinity of ranging or ‘minim’ letters such as m, n, and u.

Indeed, the spelling brayn is easier to read in Middle English writing, due to how the letters looked.