seek() and tell() in text mode, file handling python

Question

I learnt that f.tell() returns an integer giving the file object’s current position in the file represented as number of bytes from the beginning of the file when in binary mode and an opaque number when in text mode. but here tell() is changing the output anyone know how?

with open(r"C:\Users\Harshit\Desktop\file15.txt","w+") as f:
    a=f.read()
    f.write("hello1")
    f.write("hello2last")
    f.seek(0,0)
    f.write("HELLO3")
    f.write("HELLO3")
    print(f.read(1))
    f.write("added")
    f.seek(0,0)
    f.write("55555")
print(a,type(a))

output:

555551hello2lastHELLO3HELLO3added

also using tell() before read(1) i.e print(f.tell(),f.read(1)) changes output to :

555553HELLO3lastadded

whereas in binary mode it gives the perfect output as expected

555553HELLO3ladded

why i m getting faulty outputs in text mode?