turtle goes to much down

title: turtle goes to much down

i programmed my turtle that it shall dig down and then go one block down. But when there are two blocks space on its path it will take them in once. So for every two blocks space it will get one block less back up.
I don't know wheather it is a bug or a programmmistake. I'm playing on minecraftversion 1.4.7 and here is my code

function load(str)
		lade = fs.open(str,"r")
		geles = lade.readLine()
		lade.close()
end
	
function save(str, text)
		speich = fs.open(str,"w")
		speich.write(text)
		speich.close()  
end
	
function runter()
		while (bedrock == false) do
		    turtle.digDown()
		    os.sleep(0.1)
		    turtle.down()
		    y=tonumber(y)+1
		    save("y", y)
		    print(y)
		    if (turtle.digDown() == false and turtle.down() == false) then
			    bedrock = true
		    end
		end
		zustand="rauf"
		save("zustand", zustand)
end
	
function rauf()
		zustand="rauf"
		save("zustand",zustand)
		while (tonumber(y) > 0) do
				turtle.up()
				y=tonumber(y)-1
			    print(y)
				save("y",y)
		end
		turtle.select(1)
		turtle.placeUp()
		for i=3,16 do
		    turtle.select(i)
			turtle.dropUp()
	    end
		turtle.select(1)
		turtle.digUp()
		turtle.select(2)
		turtle.placeUp()
		turtle.suckUp()
		turtle.refuel()
		turtle.digUp()
end

load("start")
start=geles
print(start)
if (start ~= "true") then
		print("Willst du starten? j/n")
		input = read()
		if (input == "j") then
				start="true"
				zustand = "runter"
				y="0"
				bedrock = false
				save("bedrock",bedrock)
				save("y",y)
				save("zustand",zustand)
				save("start",start)
		end
		if (input=="n")then
				start="0"
				save("start",start)
		end
end
	
while (start == "true") do
		shell.run("clear")
		load("zustand")
		zustand = geles
		print(zustand)
		if (zustand == "runter") then
				load("y")
				y=geles
				runter()
		end
		if (zustand == "rauf") then
				load("y")
				y=geles
				rauf()
		end
		start=0
		save("start",start)
end
print("end")

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