Mismatch get_coherence_per_topic and get_coherence for single topic

[silviatti]

Problem description

Hi! I am using Gensim to compute the NPMI coherence for each of my topics. I used the method get_coherence_per_topic() and also get_coherence() (in this case, just passing a list with a single topic), and I noticed that the coherences per topic do not match with the ones returned by get_coherence() of the corresponding topics. In my understanding, the NPMI of a topic should be independent of the number of topics or of the other input topics.
This happens also with the other c_* coherences, not with the UMASS version.

Thank you!

Steps/code/corpus to reproduce

from gensim.test.utils import common_texts, common_dictionary
from gensim.models.ldamodel import LdaModel
from gensim.models.coherencemodel import CoherenceModel

topics = [
    ['human', 'computer', 'system', 'interface'],
    ['graph', 'minors', 'trees', 'eps']
]

cm = CoherenceModel(topics=topics, texts=common_texts, coherence='c_npmi', 
                    dictionary=common_dictionary)
coherence = cm.get_coherence_per_topic()  
print(coherence) # got [0.23583958321789514, -0.24456941091456053]

cm_topic0 = CoherenceModel(topics=[topics[0]], texts=common_texts, 
                           coherence='c_npmi', dictionary=common_dictionary)
coherence_topic0 = cm_topic0.get_coherence()  
print(coherence_topic0) # expect this to be == coherence[0] but got -0.14624062517782566

cm_topic1 = CoherenceModel(topics=[topics[1]], texts=common_texts, 
                           coherence='c_npmi', dictionary=common_dictionary)
coherence_topic1 = cm_topic1.get_coherence()  
print(coherence_topic1) # expect this to be == coherence[1] but got -0.31633310918174923

Versions

Linux-5.4.104+-x86_64-with-Ubuntu-18.04-bionic
Python 3.7.10 (default, May 3 2021, 02:48:31)
[GCC 7.5.0]
Bits 64
NumPy 1.19.5
SciPy 1.4.1
gensim 3.8.3
FAST_VERSION 1