Why the systems disagree
Each tradition slices time differently. Celtic tree signs follow a lunar calendar of thirteen months, Egyptian assignments follow deity associations by date, and east Asian systems mostly work by year rather than by day. Running one birth date through several is the quickest way to see that no two are measuring the same thing.
Some of these systems are ancient and well documented. Others were assembled much more recently from older material, and we say which is which rather than dressing a modern arrangement up as antiquity.
Reading a sign from another tradition
A sign from a tradition you did not grow up in is best read as a fresh description rather than a competing verdict. The interesting part is usually where it agrees with something you already suspected.
The western zodiac library covers the twelve signs most readers arrive knowing.