Because the bit pattern that means 150 in any larger integer type means -106 in an i8. 150 in binary is 10010110, and i8 means the 8th bit is the sign bit of a two's complement number, so 10010110 = negative 1101001 - 1 = -106.
You declared x to have type i8, a signed 8-bit integer. In this representation the bit pattern 10010110 has the value -106. Bit 7 has the value -128 so that you can represent negative numbers, so -128 + 16 + 4 + 2 = -106.
Now in general 150 is also represented by the same bit pattern, but then it means 128 + 16 + 4 + 2. That is, in other integer types bit 7 is not used to represent negative numbers, so it can have the value 128.
This is basic twos complement math, and the same as in for example C and Java.
If you want to use an 8-bit number, but be able to represent negative numbers, the positive range (0…255) has to be cut in half to make space for negative numbers.
The usual way to do this is twos-complement, as the other posters said, so that the highest-most bit (bit 7 when counting from 0) gets a “weight” of -128 instead of 128. This means that the new range of the type is (-128…127).
@starblue is counting from the least significant (rightmost) bit and considering it to be "bit 0". So "bit 7" is the leftmost bit (as written) in the 8-bit value.
I know you mean well, but I would like to encourage you to use a more gender-neutral term (e.g. your use of "mate" seems fine to me), especially when gender is not obvious from someone's profile. I don't believe we have any explicit guideline about that for the forum, but using the wrong term is an easy way to accidentally make someone feel uncomfortable.
(Plus, there's some negative association with the specific term "bro" in some circles; e.g. the term "brogrammer".)