Why are there holes in the Unicode table?

Question

Given this area of the Unicode table, for instance:

  ...
𝑎    U+1D44E Dec:119886       MATHEMATICAL ITALIC SMALL A 𝑎
𝑏    U+1D44F Dec:119887       MATHEMATICAL ITALIC SMALL B 𝑏
𝑐    U+1D450 Dec:119888       MATHEMATICAL ITALIC SMALL C 𝑐
𝑑    U+1D451 Dec:119889       MATHEMATICAL ITALIC SMALL D 𝑑
𝑒    U+1D452 Dec:119890       MATHEMATICAL ITALIC SMALL E 𝑒
𝑓    U+1D453 Dec:119891       MATHEMATICAL ITALIC SMALL F 𝑓
𝑔    U+1D454 Dec:119892       MATHEMATICAL ITALIC SMALL G 𝑔
𝑖    U+1D456 Dec:119894       MATHEMATICAL ITALIC SMALL I 𝑖 # what?!
𝑗    U+1D457 Dec:119895       MATHEMATICAL ITALIC SMALL J 𝑗
𝑘    U+1D458 Dec:119896       MATHEMATICAL ITALIC SMALL K 𝑘
𝑙    U+1D459 Dec:119897       MATHEMATICAL ITALIC SMALL L 𝑙
𝑚    U+1D45A Dec:119898       MATHEMATICAL ITALIC SMALL M 𝑚
𝑛    U+1D45B Dec:119899       MATHEMATICAL ITALIC SMALL N 𝑛
𝑜    U+1D45C Dec:119900       MATHEMATICAL ITALIC SMALL O 𝑜
  ...

I would naturally expect u+1d455 to be MATHEMATICAL ITALIC SMALL H. But it seems not defined on any table I look around.

Why are there holes in Unicode table? (also U+1d49d, u+1d53a, etc.)
Is there any way I can fill them?

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[EDIT]: These links do state:

The "holes" in the alphabetic ranges are filled by previously defined characters in the Letter like Symbols block shown below.

and

The Unicode Consortium adds new codepoints to the standard all the time. Visit their website to find out about pending codepoints and whether this one is in the pipe. The following table shows typical representations of how the codepoint would look, if it existed. This may help you when debugging, but is not of real use otherwise.

But I just... don't understand what they mean :\

Answer 1

From the comments (cheers guys), I have learnt that these holes are due to some characters being already assigned in Unicode when the whole alphabet had been added.

For instance: before U+1D4* MATHEMATICAL ITALIC SMALL * identifiers were defined, ℎ was already known in the table as

ℎ    U+210E Dec:008462        PLANCK CONSTANT ℎ # here it is

So in order to keep consistency in numbering but NOT to duplicate ℎ id, a hole has been inserted at U+1D455 position.

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Similarly, ℬ is known as U+212C SCRIPT CAPITAL B rather than U+1D49D - - - reserved in the MATHEMATICAL SCRIPT CAPITAL letters family.

Similarly, ℂ from MATHEMATICAL DOUBLE-STRUCK CAPITAL letters family is not U+1D53A because it was already known as U+2102 DOUBLE-STRUCK CAPITAL C.

This was a difficult choice, having to deal with retro-compatibility, consistency and reliability altogether :)