Your Vedic Sun sign may be different from your Western Sun sign because Jyotish is a visual system that uses the sidereal zodiac, whereas Western Astrology is a symbolic system that uses the tropic zodiac. I know what most of you are thinking - what does this mean?
Well, Western Astrology uses the first day of Spring to symbolize new beginnings. So, the Western system says that on the first day of Spring every year, the Sun enters the first sign of the Zodiac, Aries. From that date, the rest of the planetary movements are calculated. However, if you looked up in the sky, the Sun actually entered Aries on April 14th this year, not on the first day of Spring (March 20th). This is because as our Earth rotates, it wobbles a little bit, causing the the Sun to enter Aries at a slightly later time every year. For reference, the last time the Sun first entered Aries on the first day of Spring was about 1500 years ago.
Jyotish, on the other hand, is a visual system, so it uses fixed stars to calibrate the date and time when all the heavenly bodies move across signs. These have been verified through modern astronomical observations. Said another way, if you looked into the sky when you were born, your Sun would be in your Jyotish Sun sign, but may or may not be in your Western Sun sign.
Having said all this, Western Astrology works well in isolation, and so my general advice is to not compare astronomical details across astrology systems. Both systems should tell you similar things, and generally the universe guides you to the astrologer that can tell you what you need to know.