From Pinnan to Heian and Naifanchi to Tekki Shodan, Master Funakoshi systemised the katas he knew to make the kata we know today. Several other influential karateka have added others over the years including the second and third Tekki katas created by Master Itosu (a teacher of Master Funakoshi) who is credited with developing both of the other Tekki Kata (Nidan and Sandan).
Katas are usually done stepping forward (as this is easier to do when learning moves) but applications (Bunkai and Oyo) can be done stepping backwards, sidewards, on the diagonal angles or even a double move (back and then forwards in one move). This is where the principles of go-no-sen (dont give ground), tai-no-sen (Give ground and then attack half way through the attack) and sen-no-sen (give ground until attack is completed and then retaliate) come into play and these applications can be done according to each of these ideas.
It is important for the understanding that bunkai (application of kata) can be done at various levels. When learning kata, the moves shown to kyu grades (and then practised when the kata is familiar to the student) are a simplistic view using "basic" techniques that are workable to a certain degree. When the student has progressed to dan grade and can perform the kata to a proficient level - the more realistic applications are shown by senior instructors. This is not to hold the kyu grade students back but is to make sure that the underlying principles of karate are known before these potentially dangerous applications are attempted. Most katas were developed when karate was a jutsu (fighting) form rather than a do (way) form and includes nerve striking, throwing, joint locking and breaking techniques. Kyusho Jutsu / Atemi Waza (pressure point attacking) is utilised in kata and can result in nerve damage / unconciousness / internal organ damage / death and therefore is only taught to dan grade students.
So with this in mind, the only students that will be shown "full" bunkai are mature and sensible adult dan grades to ensure that this knowledge is only known and not used (except in the defense of your own life!) on others.
Finally, remember that the bunkai we show you is only one way of doing it, different body sizes and shapes will make certain techniques work in different ways, we show you the way to work out the bunkai for yourselves (and trying it on others in a safe environment to see if it will work) at the level relevant to your grade.
Taking bunkai to the next level of adapting the moves to fit different situations is called Oyo (also known as close quarter kumite). This is only taught to senior grades, once bunkai is fully understood and the student is able to take the next step themselves and be able to work out what kata means to them.