How come two people who are both Christians can have different beliefs about issues such as abortion?

Christianity is a religion with different denominations - which means that there are different churches and beliefs even within Christianity itself. Different denominations and individual Christians themselves can decide upon which teachings and parts of the Bible they wish to focus on and apply in their lives. For example, with issues such as abortion, it depends on what a particular Christian believes about how life begins. Some Christians, such as Catholics, believe that life begins at the moment of conception - that is when the sperm meets the egg. As a result, they believe abortion is always wrong because abortion at any stage would be killing a human being.
A more liberal Christian might argue that abortion is wrong, but that the wrong suffered in forcing a woman to have a baby against her wishes is more severe. In that case, they might argue that abortion is generally wrong, but that it can be justified when balanced against the idea that women ought to have the final choice over what they do with their body. These Christians may use Jesus' Bible teachings such as 'love thy neighbour' or 'do to others as you would have them do to you' to support the idea that abortion can sometimes be justified under Christianity.