Images with 256 colors are not compressed with JPEG in exported PDF

I'm confused about how InDesign compresses images in pdfs.
I have to submit 15 photos to a photography contest, so I put these 15 images (each image in its own page) in an InDesign document that has no more than these 15 pages. All the images have 2000 pixels in the larger side and have in average 689 KB.
In the Export to pdf dialog, I choose "Smallest file size" preset and JPEG compression (not JPEG (Automatic)) for both "Color images" and "Grayscale images".
However the resulting pdf size is aprox. 26 MB! I expect the pdf to be a little more than 689 KB * 15 = 10335 KB or 10 MB.
An inspection of the pdf with pdfimages, a tool from a pdf library called Poppler, reveals that only 3 out the 15 images are compressed with jpeg.

$ pdfimages -list mypdf.pdf 
page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
   1     0 image    2000  1337  index   1   8  image  no        78  0    72    72 2195K  84%
   2     1 image    2000  1337  index   1   8  image  no         4  0    72    72 2155K  83%
   3     2 image    2000  1333  icc     3   8  jpeg   no         8  0    72    72  120K 1.5%
   4     3 image    1500  2000  icc     3   8  jpeg   no        12  0    72    72 98.3K 1.1%
   5     4 image    2000  1333  index   1   8  image  no        16  0    72    72 1919K  74%
   6     5 image    1334  2000  index   1   8  image  no        20  0    73    73 2058K  79%
   7     6 image    1500  2000  index   1   8  image  no        24  0    72    72 1781K  61%
   8     7 image    2000  1332  index   1   8  image  no        28  0    72    72 1943K  75%
   9     8 image    2000  1333  index   1   8  image  no        33  0    72    72 2168K  83%
  10     9 image    2000  1337  index   1   8  image  no        37  0    72    72 2354K  90%
  11    10 image    1500  1998  index   1   8  image  no        41  0    72    72 2080K  71%
  12    11 image    1500  2000  index   1   8  image  no        45  0    72    72 1869K  64%
  13    12 image    2000  1339  index   1   8  image  no        49  0    72    72 2465K  94%
  14    13 image    2000  1332  icc     3   8  jpeg   no        53  0    72    72  120K 1.5%
  15    14 image    1337  2000  index   1   8  image  no        57  0    72    72 2381K  91%

Inspecting each image in detail, I found that the 3 images that were compressed with JPEG have more than 256 unique colours, while the others have exactly 256 unique colours. Also notice that the images that have 256 unique colours have an "Indexed color space" in the pdf.

Using LibreOffice writer, a word processor commonly used in Linux distributions, I could export to a 8,2 MB pdf.

So my questions.
How can I instruct InDesign to compress with JPEG all the images and not only ones that have more than 256 colours?
Is this an expected behaviour of the pdf compression process in InDesign?

Thank you all in advance.