According to astrology, the Sun should currently be in my sign but Sky Guide shows it in a different position along the zodiac. Why?

While it is true that astronomy and astrology share similar roots, they diverged long ago. Ancient astrologers were not familiar with the concept of precession, whereby the axis of the Earth slowly wobbles over the millennia, causing the Sun to trace out a different path across the celestial sphere. Simply put, astrology does not accurately predict the present day position of the Sun relative to the stars.