Ruby perlin simplex issue

perlin_test.rb

require 'perlin'

perl = Perlin::Generator.new 226, 0.25, 2

x = 1.1867
y = 12.90625044

mod = 0.0000004

p = perl[x, y]
right = perl[x + mod, y]
down = perl[x, y + mod]
left = perl[x - mod, y]
up = perl[x, y - mod]

puts "player height: " + p.to_s
puts "right: " + right.to_s + ", diff: " + ((p - right)*100).to_s
puts "down: " + down.to_s + ", diff: " + ((p - down)*100).to_s
puts "left: " + left.to_s + ", diff: " + ((p - left)*100).to_s
puts "up: " + up.to_s + ", diff: " + ((p - up)*100).to_s

# Output:
# player height: 0.4471040368080139
# right: 0.447103887796402, diff: 1.4901161193847656e-05
# down: 0.36923521757125854, diff: 7.786881923675537
# left: 0.4471041262149811, diff: -8.940696716308594e-06
# up: 0.4471040368080139, diff: 0.0